Methods of recycling and reshaping thermosetting polymers and composites thereof

ABSTRACT

Various methods of reshaping and recycling thermoset polymers and composites containing thermoset polymers are provided. The methods involve the bond exchange reaction of exchangeable covalent bonds in the polymer matrix with a suitable small molecule solvent in the presence of a catalyst. In some aspects, the methods are applied to a carbon fiber reinforced polymer or a thermoset polymer where the thermoset polymer matrix includes a plurality of ester bonds. Using a small molecule alcohol, the methods provide for recycling one or both of the carbon fiber and the polymer, for welding two surfaces, or for repairing a damaged surface in the materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, co-pending U.S.provisional application entitled “Recycling Of Covalent AdaptablePolymers And Composites By Using Small Molecule Solvent” having Ser. No.62/357,642, filed Jul. 1, 2016.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under award CMMI-1404627awarded by the National Science Foundation. The government has certainrights in the invention.

TECHNICAL FIELD

The present disclosure generally relates to crosslinked polymers andpolymer composites and methods of processing crosslinked polymers andpolymer composites, and more particularly to methods of recycling andreshaping thermosetting polymers and composites containing thermosettingpolymers.

BACKGROUND

Thermosetting polymers are a group of polymers extensively used in avariety of high-temperature applications, such as coatings, adhesives,composites, electrical insulators and car bodies. See, e.g. the variousdiscussions in Epoxy Polymers. 2010, Wiley-VCH Verlag GmbH & Co. KGaA.They generally exhibit better thermomechanical properties and chemicalresistance than thermoplastic materials due to their chemicallycrosslinked nature. However, reshaping and recycling these materials isinherently difficult; reprocessing thermosetting polymer waste can leadto both environmental and economic issues.

Thermosetting polymers are also used in a variety of compositematerials. For example, carbon fiber reinforced polymer (CFRP)composites, with their superior combination of stiffness, strength, andlight weight, have been leading contenders in various applicationsranging from aerospace to ground transportation to sporting goods. Withthe increasing amount of CFRPs being used, their waste materials,including off-cuts during manufacture and end-of-life components, arereaching a significant level, which has raised environmental andeconomic awareness for the need to recycle the CFRP waste. Fullyrecycling CFRP composites involves the recycling of both the polymermatrix and the carbon fibers. This presents some significant challenges.

First, the thermosetting polymers that are routinely applied as bindersin the composite system are difficult to recycle. Second, completerecycling of CFRPs requires stripping away the polymer to reclaim theembedded fibers, as the high performance reinforcement is the mostexpensive part in CFRP. Currently, a variety of technologies have beeninvestigated, which can be classified into two categories: mechanicalrecycling and thermo-chemical recycling.

The mechanical recycling of CFRPs involves mechanical comminution on thecomposite by shredding, crushing, milling, or other similar mechanicalprocess The resulting scrap pieces can then be segregated by sievinginto powdered products (rich in thermoset powder) and fibrous products(rich in fibers). Typical applications for mechanically-recycledcomposites include their re-incorporation in new composites as fillerand use in the construction industry (e.g. as fillers for artificialwoods or asphalt). However, these products represent low-valueapplications because mechanical recycling does not recover individualcarbon fibers.

For more efficient fiber reclamation, thermo-chemical recyclingtechniques are routinely applied, using thermal (e.g. pyrolysis) orchemical processes to break down the thermoset matrix. For example, inthe previous work of Jiang and Rudd et al., the epoxy resin matrix wasoxidized into gas products, light aliphatic hydrocarbons and aromatichydrocarbons at 550° C. Clean carbon fiber was then elutriated withabout 80% of the original strength. A similar thermal oxidative processwas used by Jody et al. where the recycled fibers retained 95% of themodulus and 50% of the original strength. Some other recyclingtechniques in this realm rely on the use of supercritical chemicals todissolve the polymer matrix. For example, Liu et al. used nitric acid tobreak down epoxy fiber composite at 90° C. for 20-100 h. The loss oftensile strength was about 1.1%. Pinero-Hernanz et al. usedsupercritical water to recycle carbon fiber/epoxy resin composites. Therecycling was conducted at a temperature of 250-400° C., a pressure of4-27.0 MPa and a reaction time of 1-30 min. The removal of epoxy wasabout 95% with the addition of catalysts in the supercritical water. Thetensile strength of the recycled fiber was reduced to 90-98% of thevirgin fiber. The carbon fibers are vulnerable to length shorting andproperty degradation in the aforementioned recycling methods. Thethermo-chemical recycling techniques usually involve the use ofsupercritical chemicals, which are inconvenient to handle, andeconomically unfavorable in large scale engineering applications.Besides, during the recycling, only the carbon fibers are reclaimedwhile the polymer matrix is sacrificed without recovering usefulchemicals. This issue has limited the application of a recyclingtechnique, especially when the composites are manufactured incombination with other materials.

The recent development of reversible networks provides excitingopportunities for reshaping and recycling thermoset polymers. Forexample, the so-called covalent adaptable networks (CANs), or dynamiccovalent networks (DCNs), can alter the arrangement of networkconnections by bond exchange reactions (BERs). See, e.g. Bowman, C. N.and C. J. Kloxin, Angewandte Chemie-International Edition, 2012. 51(18):p. 4272-4274; Kloxin, C. J., et al., Macromolecules, 2010. 43(6): p.2643-2653; Long, R., et al., Journal of the Mechanics and Physics ofSolids, 2013. 61(11): p. 2212-2239; Wojtecki, R. J., et al., NatureMaterials, 2011. 10(1): p. 14-27; and Rowan, S. J., et al., AngewandteChemie-International Edition, 2002. 41(6): p. 898-952.

However, most of the existing CAN polymers require some specialprocessing conditions that are inconvenient in practical applications(Taynton et al., Adv. Mater., 28: 2904-2909). For example, during thesurface welding of the aforementioned CANs, an external pressure rangingfrom hundreds to thousands of kPas is usually preferred to gain a goodcontact on the interface. A welding pressure is also desired toreprocess and recycle thermosetting polymers from the powder state,where numerous interfaces are involved. Additionally, in some CANsystems such as DGEBA epoxy-fatty acid, the concentration of interfacialactive groups would decay when idling in the atmosphere via oxidation orself-annihilation, which consequently requires special treatment (e.g.additional cutting or polish) on the surface in order to regainsufficient active groups for welding.

There remains a need for improved methods of recycling and reshaping ofthermosetting polymers and composites containing thermosetting polymersthat overcome the aforementioned deficiencies.

Furthermore, there remains a need for improved methods of recycling andreshaping of carbon fiber composites that overcome the aforementioneddeficiencies.

In addition, there remains a need for improved recycled thermosettingpolymers and composites thereof that overcome the aforementioneddeficiencies.

In particular, there remains a need for improved recycled carbon fibersand recycled carbon fiber composites that overcome the aforementioneddeficiencies.

SUMMARY

In various aspects, methods are provided that overcome one or more ofthe aforementioned problems. Methods are provided for repairing athermoset polymer or a composite thereof. Methods are also provided forwelding a surface of a thermoset polymer or a composite thereof to adifferent surface containing the thermoset polymer or a compositethereof. Methods of recycling thermoset polymers and composites thereofare also provided. The methods can allow for the recovery of one or bothof the thermoset polymer and the reinforcing material.

In various aspects, methods are provided for recycling a carbon fiberreinforced polymer. The carbon fiber reinforced polymer can include (i)a thermoset polymer matrix having a plurality of ester bonds and (ii)carbon fiber dispersed within the thermoset polymer matrix. The methodscan include washing the carbon fiber reinforced polymer in a smallmolecule alcohol in the presence of a catalyst at a first elevatedtemperature for a first period of time sufficient to dissolve thethermoset polymer matrix and reclaim the carbon fiber; wherein the smallmolecule alcohol has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of ester bonds; andwherein the boiling point of the small molecule alcohol is below athermal decomposition temperature of the carbon fiber. The methods canalso include evaporating the small molecule alcohol at a second elevatedtemperature for a second period of time to re-polymerize the thermosetpolymer matrix. In some aspects, the method further includes recombiningthe small molecule alcohol and the reclaimed carbon fiber prior to theevaporating step, wherein the re-polymerizing of the thermoset polymermatrix produces a recycled carbon fiber reinforced polymer including (i)the thermoset polymer matrix having the plurality of ester bonds and(ii) the reclaimed carbon fiber dispersed within the thermoset polymermatrix.

In various aspects, methods are provided for recycling a thermosetpolymer or a composite thereof, wherein the thermoset polymer orcomposite thereof includes a thermoset polymer matrix having a pluralityof ester bonds. The method can include washing the thermoset polymer ora composite thereof in a small molecule alcohol in the presence of acatalyst at a first elevated temperature for a first period of timesufficient to dissolve the thermoset polymer matrix; and evaporating thesmall molecule alcohol at a second elevated temperature for a secondperiod of time to re-polymerize the thermoset polymer matrix; whereinthe small molecule alcohol has a boiling point above a bond exchangereaction (BER) activation temperature for the plurality of ester bonds.In some aspects, a composite is recycled, and the methods includereclaiming the reinforcing material from the thermoset polymer matrixprior to re-polymerizing the thermoset polymer matrix.

In various aspects, methods are provided for repairing a surface of athermoset polymer or a composite thereof. The thermoset polymer orcomposite thereof can include a thermoset polymer matrix having aplurality of ester bonds, and the surface can have an imperfection. Themethods can include (A) applying a powder of the thermoset polymer tothe surface of the thermoset polymer or composite thereof, (B)contacting the surface of the thermoset polymer or composite thereof andthe powder with a small molecule alcohol in the presence of a catalystat a first elevated temperature for a first period of time sufficient todissolve the powder and at least a portion of the thermoset polymermatrix at the surface of the thermoset polymer or composite thereof, and(C) evaporating the small molecule alcohol at a second elevatedtemperature for a second period of time sufficient to re-polymerize thethermoset polymer matrix incorporating the thermoset polymer from thepowder to repair the imperfection; wherein the small molecule alcoholhas a boiling point above a bond exchange reaction (BER) activationtemperature for the plurality of ester bonds.

In some aspects, a method is provided for chemically welding a firstsurface to a second surface, wherein both the first surface and thesecond surface include a thermoset polymer matrix having a plurality ofester bonds. The methods can include (A) contacting the first surfaceand the second surface with a small molecule alcohol in the presence ofa catalyst at a first elevated temperature for a first period of time;(B) contacting the first surface and the second surface to form aninterface; and (C) evaporating the small molecule alcohol to polymerizethe thermoset polymer matrix at the interface; wherein the smallmolecule alcohol has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of ester bonds. Theinterface can have an interfacial fracture energy of about 1200 J/m² to5000 J/m².

In some aspects, a method is provided for recycling a carbon fiber froma carbon fiber reinforced polymer, wherein the carbon fiber reinforcedpolymer includes (i) a thermoset polymer matrix having a plurality ofexchangeable covalent bonds and (ii) a carbon fiber dispersed within thethermoset polymer matrix. The method can include (A) washing the carbonfiber reinforced polymer in a small molecule solvent capable of a bondexchange reaction with the plurality of exchangeable covalent bonds inthe presence of a catalyst at a first elevated temperature for a firstperiod of time sufficient to dissolve the thermoset polymer matrix andreclaim the carbon fiber; wherein the small molecule solvent has aboiling point above a bond exchange reaction (BER) activationtemperature for the plurality of exchangeable covalent bonds; andwherein the boiling point of the small molecule solvent is below athermal decomposition temperature of the carbon fiber.

In some aspects, a method is provided for recycling a thermoset polymeror a composite thereof, wherein the thermoset polymer or compositethereof includes a thermoset polymer matrix having a plurality ofexchangeable covalent bonds. The methods can include (A) washing thethermoset polymer or a composite thereof in a small molecule solvent inthe presence of a catalyst at a first elevated temperature for a firstperiod of time sufficient to dissolve the thermoset polymer matrix; and(B) evaporating the small molecule solvent at a second elevatedtemperature for a second period of time to re-polymerize the thermosetpolymer matrix; wherein the small molecule solvent has a boiling pointabove a bond exchange reaction (BER) activation temperature for theplurality of exchangeable covalent bonds.

In some aspects, a method is provided for repairing a surface of athermoset polymer or a composite thereof, wherein the thermoset polymeror composite thereof includes a thermoset polymer matrix having aplurality of exchangeable covalent bonds, and wherein the surface has animperfection. The method can include (A) applying a powder of thethermoset polymer to the surface of the thermoset polymer or compositethereof, (B) contacting the surface of the thermoset polymer orcomposite thereof and the powder with a small molecule solvent in thepresence of a catalyst at a first elevated temperature for a firstperiod of time sufficient to dissolve the powder and at least a portionof the thermoset polymer matrix at the surface of the thermoset polymeror composite thereof, and (C) evaporating the small molecule solvent ata second elevated temperature for a second period of time sufficient tore-polymerize the thermoset polymer matrix incorporating the thermosetpolymer from the powder to repair the imperfection; wherein the smallmolecule solvent has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of exchangeable covalentbonds.

In some aspects, a method is provided for chemically welding a firstsurface to a second surface, wherein both the first surface and thesecond surface include a thermoset polymer matrix having a plurality ofexchangeable covalent bonds. The method can include (A) contacting thefirst surface and the second surface with a small molecule solvent inthe presence of a catalyst at a first elevated temperature for a firstperiod of time; (B) contacting the first surface and the second surfaceto form an interface; and (C) evaporating the small molecule solvent topolymerize the thermoset polymer matrix at the interface; wherein thesmall molecule solvent has a boiling point above a bond exchangereaction (BER) activation temperature for the plurality of exchangeablecovalent bonds. The interface can have an interfacial fracture energy ofabout 1200 J/m² to 5000 J/m².

The methods can use a small molecule solvent. In various aspects, thesmall molecule solvent can be a small molecule alcohol, a small moleculethiol, or a small molecule amine. In some aspects the plurality ofexchangeable covalent bonds are ester bonds and the small moleculesolvent is a small molecule alcohol. In some aspects, the plurality ofexchangeable covalent bonds are sulfide bonds and the small moleculesolvent is a small molecule thiol. In some aspects, the plurality ofexchangeable covalent bonds are imide bonds and the small moleculesolvent is a small molecule amine.

In some aspects, the small molecule solvent is a small molecule alcohol.The small molecule alcohol can be a polyol. The small molecule alcoholcan be a diol, a triol, or a polyol. The small molecule alcohol can beethylene glycol, propylene glycol, 1,3-butanediol diethylene glycol,2-ethyl-hexanol, or benzyl alcohol. The small molecule alcohol can havea boiling point of about 160° C. to 180° C. The small molecule alcoholcan have a molecular weight of about 50 g/mol to 150 g/mol. The boilingpoint of the small molecule alcohol can be about 250° C. or less.

In some aspects, the small molecule solvent is a small molecule thiol.The thiol can be a mono-thiol or poly-thiol solvents. The small moleculethiol can be 2-mercaptoethanol, dithiothreitol,3-mercaptopropane-1,2-diol, butanethiol, or 1,3-propanedithiol.

In some aspects, the small molecule solvent is a small molecule amine.The small molecule amine can be a mono-amine or poly-amine solvent. Thesmall molecule amine can be hexylamine, 2-ethyl-1-hexylamine,2-amino-5-methylhexane, ethylenediamine, or diethylenetriamine.

The thermoset polymer can include a plurality of exchangeable covalentbonds. In some aspects, the exchangeable covalent bonds are esters andthe thermoset polymer matrix includes an anhydride cured epoxy, anunsaturated polyester, or a combination thereof. In some aspects, theexchangeable covalent bonds are sulfide bonds and the thermoset polymermatrix includes a dithiodianiline cured epoxy, a disulfide based epoxy,or a combination thereof. In some aspects, the exchangeable covalentbonds are imide bonds and the thermoset polymer matrix includes apolyimine crosslinked by tris(2-aminoethyl)amine with a spacer monomerof diethylene triamine, 3thylene diamine, or a combination thereof.

The methods can include a variety of catalysts. The catalyst can bepresent in any amount sufficient to catalyze the bond exchange reaction.In some aspects, the catalyst is present in an amount from about 2 mol-%to about 10 mol-%. In some aspects, the catalyst is atransesterification such as a lead oxide, a lead sulfide, a leadhydroxide, a plumbite, a plumbate, a lead carbonate, a copper compound,a silver compound, a gold compound, a zinc compound, a cadmium compound,an iron compound, a cobalt compound, a salt thereof, or a combinationthereof. In some aspects, the catalyst is a metal-ligand complex,wherein the metal-ligand complex comprises a metal selected from thegroup consisting of Cu, Li, Zn, Cd, Fe, and Co; and wherein themetal-ligand complex comprises one or more ligands independentlyselected from the group consisting of acetylacetone, a halide, and acombination thereof. In some aspects, the catalyst is an organic basesuch as 4-dimethylaminopropyridine, 1,8-diazabicyclo[5,4,0]undec-7-ene,diethylenetriamine, triethylamine, triphenylphosphine, or n-butylphosphate. In some aspects, the catalyst is a Brønsted acid or Lewisacid.

The methods can include heating to a first elevated temperature for afirst period of time and/or to a second elevated temperature for asecond period of time. In various aspects, one or both of the firstelevated temperature and the second elevated temperature areindependently about 160° C. to 200° C. In various aspects, one or bothof the first period of time and the second period of time areindependently about 2 hours to 6 hours.

In some aspects, the method are applied to a composite of a thermosetpolymer, and the thermoset polymer includes a reinforcing materialdispersed within the thermoset polymer matrix. The reinforcing materialcan be a material such as a glass fiber, a carbon fiber, an aramidfiber, a boron fiber, a graphite, or a combination thereof. In someaspects, the reinforcing material is a carbon fiber and the composite isa carbon fiber reinforced polymer. In some aspects, the reinforcingmaterial has a thermal decomposition temperature of about 500° C. orless. The reinforcing material can have a structure such as a continuousfiber, a cloth, a fabric, a yarn, or a tape.

Other systems, methods, features, and advantages of methods of recyclingand reshaping thermosetting polymers and composites thereof will be orbecome apparent to one with skill in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional systems, methods, features, and advantages be includedwithin this description, be within the scope of the present disclosure,and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be readily appreciatedupon review of the detailed description of its various embodiments,described below, when taken in conjunction with the accompanyingdrawings.

FIGS. 1A-1B are schematic graphs showing the polymerization process andnetwork adaptiveness. FIG. 1A is a schematic of the polymerizationreaction of epoxy thermoset with dicarboxylic and tricarboxylic groupsfrom fatty acid. FIG. 1B is a schematic of the BER in a representativepolymer network, showing how the network topology can be rearranged whensuch reactions occur in a large amount through (I) the network before aBER; (II) intermediate state of the exchange reaction; (III) and afterthe BER.

FIGS. 2A-2C are schematic graphs showing the exemplary dissolution andrepolymerization mechanisms. FIG. 2A is a schematic of the dissolutionof the epoxy thermoset in ethylene glycol (EG) solvent in an enclosed(sealed) environment. FIG. 2B is a schematic of the repolymerization ofthe dissolved epoxy thermoset in an open-air environment. FIG. 2C is aschematic of the view illustrating the EG-assisted dissolution andrepolymerization of epoxy thermosets.

FIGS. 3A-3C are schematic illustrations of exemplary pressure-freewelding and damage repair of adaptable thermosets. FIG. 3A is aschematic demonstrating an exemplary process for welding of partiallydissolved CANs on the interface: (I) the epoxy samples are immersed inEG; (II) the samples are taken out of solvent; (III) the two samples arestacked together; (IV) producing the welded material. FIG. 3B is aschematic demonstrating an exemplary process for the welding usingEG-epoxy glue: (I) the epoxy wastes are immersed in EG; (II) EG-epoxyglue is added onto surface; (III) the two materials are stackedtogether; and (IV) producing the welded material. FIG. 3C is a schematicdemonstrating an exemplary process for repairing procedure of surfacedamage via EG-assisted transesterification: (I) starting from a freshepoxy thermoset with smooth surface; (II) the epoxy thermoset can havesurface damage, e.g. via manual scratching; (III) epoxy glue is added tocover the scratches on the damaged surface; and (IV) producing the fullyrepaired epoxy thermoset with smooth surface.

FIG. 4 is a schematic demonstrating an exemplary process for thepressure-free reprocessing of adaptable thermosets: (a) epoxy thermosetis manually ground; (b) EG is mixed with the CAN powder; (c) the polymerparticles are partially dissolved on surface; and (d) the polymerparticles are reassembled after heating for 4 h at 180° C.

FIGS. 5A-5D demonstrate the dissolution tests on the epoxy thermosets.FIG. 5A is a time-lapse series of photographs demonstrating theappearance and size of an epoxy sample after being soaked in 4 g EGsolvent for different periods of time and with a catalyst concentrationin the epoxy of 5 mol % and a temperature of about 180° C. FIG. 5B is agraph of the normalized residual mass of epoxy sample as a function ofheating time at 180° C. and EG weight. FIG. 5C is a graph of thenormalized residual mass of epoxy as a function of heating time at 180°C. and catalyst concentrations. FIG. 5D is a graph of the normalizedresidual mass of epoxy as a function of heating time and temperature.The catalyst concentration in the epoxy is 5 mol %.

FIG. 6 is a graph of the accumulative mass of re-polymerized CANs as afunction of heating time at 200° C. The catalyst concentration in epoxythermoset is 5 mol %.

FIGS. 7A-7C demonstrate the thermomechanical behaviors of therepolymerized and fresh epoxy thermosets. FIG. 7A is a graph comparingthe normalized tan δ and storage modulus as a function of temperature.FIG. 7B is a graph comparing the normalized stress relaxation curves atdifferent temperatures. FIG. 7C is a graph comparing the relaxation timebetween re-polymerized and fresh CANs.

FIGS. 8A-8B demonstrate an exemplary pre-treatment welding procedure.FIG. 8A is a series of experimental imagines and microscopic opticalimages of the fresh-cut and pre-treated surfaces: (I) fresh cut samples;(II) pre-treated surfaces; (III) stacking the strips together at roomtemperature; (IV) fully welded strips. FIG. 8B is a series ofmicroscopic optical images of fully welded CANs: top view and middleview of the interface. (The arrows in the top image indicate the weldedinterfaces and the lines represent middle surface)

FIGS. 9A-9B demonstrate an exemplary EG-epoxy glue welding procedure.FIG. 9A is a series of snapshots of the process: (I) fresh samples ofCANs; (II) adding EG-epoxy onto the surface; (III) stacking the stripstogether at room temperature; and (IV) fully welded strips. FIG. 9B is aseries of microscopic views of fully welded epoxy strips from top andmiddle surfaces. (arrows indicate the welded interfaces and linesrepresent middle surface.)

FIGS. 10A-10D are graphs showing the interfacial fracture energy ofwelded epoxy samples tested by using T-peeling tests. The weldingtemperature is 180° C. FIG. 10A is a graph of the typical peeling forceas a function of displacement during the T-peeling test of the sampleheating for 180 min. FIG. 10B is a graph of the interfacial fractureenergy of welded epoxy samples via pre-treatment method. FIG. 10C is agraph of the interfacial fracture energy of welded epoxy samples viaEG-epoxy glue method. FIG. 10D is a graph comparing the interfacialfracture energy between pressure-assisted welding and the pressure-freewelding methods. The catalyst concentration in the epoxy sample is 5 mol%.

FIGS. 11A-11B are optical microscopic images showing the EG-assistedrepair of CAN surface. FIG. 11A is an optical microscope imaged epoxythermoset after scratching the surface with needles. FIG. 11B is anoptical microscope image of the repaired surface after adding epoxy gluefollowed by heating for 3 h at 180° C.

FIG. 12 depicts an exemplary process for the EG-assisted reprocessing ofepoxy thermoset from powder state (a) starting from a fresh piece ofepoxy thermoset; (b) an epoxy powder is prepared by grinding (c) theepoxy powder is mixed with EG; and (d) the reprocessed epoxy isassembled after heating at 180° C. for 4 hours. Note: the sample wastrimmed into a rectangle for good demonstration.

FIGS. 13A-13C demonstrate the mechanical properties of epoxy thermosetreprocessed at 180° C. FIG. 13A is a graph comparing the tensilestress-strain behaviors of a fresh sample with those of reprocessedsamples. The reprocessing time at 180° C. is 180 min. FIG. 13B is agraph of the tensile modulus as a function of heating time withdifferent catalyst concentrations. FIG. 13C is a graph of the ultimatestrength as a function of heating time with different catalystconcentrations.

FIGS. 14A-14B are FTIR traces to monitor the functional groups in bothdissolution and repolymerization steps. FIG. 14A are FTIR tracesdemonstrating conversions of esters (1735-1750 cm⁻¹), and epoxy COC(900-930 cm⁻¹). FIG. 14B are FTIR traces demonstrating conversion ofhydroxyls (3200-3550 cm⁻¹).

FIGS. 15A-15B demonstrate comparisons of thermomechanical propertiesbetween EG-recycled and 2E1H-recycled CANs. FIG. 15A demonstrates thedissolution/repolymerization cycle of the epoxy thermosets in 2E1H. FIG.15B is a graph comparing normalized tan δ and storage modulus as afunction of temperature between fresh, EG-recycled and 2E1H-recycledCANs.

FIGS. 16A-16B demonstrate comparisons of FTIR spectra betweenEG-recycled and 2E1H-recycled CANs in the inner surface: 1) fresh CANs,2) EG-recycled CANs (180° C., 3 h), 3) 2E1H-recycled CANs (180° C., 42h), 4) fully dissolved CANs in 2E1H (180° C., 20 h). FIG. 16Ademonstrates the conversions between esters (1735-1750 cm⁻¹) and epoxyCOC (900-930 cm⁻¹). FIG. 16B demonstrates the conversion of hydroxyls(3200-3550 cm⁻¹).

FIGS. 17A-17E demonstrate the dissolution and repolymerization of theepoxy thermosets in EG solvent. FIG. 17A is a schematic view of theformation of fatty acid linkers within the thermosetting network. FIG.17B is a schematic view of the dissolution and repolymerization of theepoxy thermoset via transesterification. FIG. 17C is a graph of thenormalized weight of epoxy as a function of soaking time. The weight ofEG is 3 g. The temperature is 180° C. FIG. 17D is a graph of theresidual weight of epoxy after being soaked for 4 h, as a function of EGweight. The temperature is 180° C. FIG. 17E is a graph of the BERinduced stress relaxation behavior between fresh epoxy (solid lines) andrecycled epoxy (dashed lines)

FIGS. 18A-18B demonstrate an exemplary process for repairing CFRPcomposite with surface damage. FIG. 18A is a schematic viewdemonstrating the exemplary process for repairing CFRP composite withsurface damage. FIG. 18B is a series of optical images demonstrating theexemplary process for repairing CFRP composite with surface damage.

FIGS. 19A-19C demonstrate an exemplary closed-loop recycling paradigmfor CFRP composite. FIG. 19A is a schematic view of the exemplaryclosed-loop recycling paradigm for CFRP composite. FIG. 19B is a seriesof photographs showing the epoxy matrix is gradually dissolved in EG at180° C., and the clean fiber fabric is reclaimed FIG. 19C is a series ofphotographs demonstrating refabricating the thermoset composite by usingthe reclaimed fiber and dissolved polymer solution.

FIGS. 20A-20E compare the fresh and reclaimed carbon fibers. FIG. 20A isa series of SEM images of the fresh carbon fiber. FIG. 20B is a seriesof SEM images of the reclaimed carbon fiber. FIG. 20C is a graph of thestress-strain curves of both fresh and reclaimed carbon fibers. FIG. 20Dis a graph of the room-temperature stress-strain behavior of fresh,recycled and repaired composite. FIG. 20E is a bar graph summary of theelastic modulus (within the first 2% stretch, top) and ultimate strengthof each generation of recycled CFRP composite (bottom).

FIGS. 21A-21B are graphs of the glass transition (FIG. 21A) andstress-strain behavior (FIG. 21B) between fresh epoxy (solid lines) andrecycled epoxy (dash lines).

DETAILED DESCRIPTION

In various aspects, methods of reshaping and recycling thermosetpolymers and composited containing thermoset polymers are provided. Themethods can use a bond exchange reaction between exchangeable covalentbonds and a suitable small molecule solvent to facilitate dissolving thepolymer matrix under conditions that preserve the integrity of thereinforcing materials in the composite and allow for recovery of boththe polymer and the reinforcing material with no or only minimaldegradation of mechanical integrity.

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, and as such may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. The skilled artisan will recognize many variants andadaptations of the embodiments described herein. These variants andadaptations are intended to be included in the teachings of thisdisclosure and to be encompassed by the claims herein.

All publications and patents cited in this specification are cited todisclose and describe the methods and/or materials in connection withwhich the publications are cited. All such publications and patents areherein incorporated by references as if each individual publication orpatent were specifically and individually indicated to be incorporatedby reference. Such incorporation by reference is expressly limited tothe methods and/or materials described in the cited publications andpatents and does not extend to any lexicographical definitions from thecited publications and patents. Any lexicographical definition in thepublications and patents cited that is not also expressly repeated inthe instant specification should not be treated as such and should notbe read as defining any terms appearing in the accompanying claims. Thecitation of any publication is for its disclosure prior to the filingdate and should not be construed as an admission that the presentdisclosure is not entitled to antedate such publication by virtue ofprior disclosure. Further, the dates of publication provided could bedifferent from the actual publication dates that may need to beindependently confirmed.

Although any methods and materials similar or equivalent to thosedescribed herein can also be used in the practice or testing of thepresent disclosure, the preferred methods and materials are nowdescribed. Functions or constructions well-known in the art may not bedescribed in detail for brevity and/or clarity. Embodiments of thepresent disclosure will employ, unless otherwise indicated, techniquesof nanotechnology, organic chemistry, material science and engineeringand the like, which are within the skill of the art. Such techniques areexplained fully in the literature.

It should be noted that ratios, concentrations, amounts, and othernumerical data can be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a numerical range of “about 0.1%to about 5%” should be interpreted to include not only the explicitlyrecited values of about 0.1% to about 5%, but also include individualvalues (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%,2.2%, 3.3%, and 4.4%) within the indicated range. Where the stated rangeincludes one or both of the limits, ranges excluding either or both ofthose included limits are also included in the disclosure, e.g. thephrase “x to y” includes the range from ‘x’ to ‘y’ as well as the rangegreater than ‘x’ and less than ‘y’. The range can also be expressed asan upper limit, e.g. ‘about x, y, z, or less’ and should be interpretedto include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ aswell as the ranges of ‘less than x’, less than y’, and ‘less than z’.Likewise, the phrase ‘about x, y, z, or greater’ should be interpretedto include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ aswell as the ranges of ‘greater than x’, greater than y’, and ‘greaterthan z’. In some embodiments, the term “about” can include traditionalrounding according to significant figures of the numerical value. Inaddition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numericalvalues, includes “about ‘x’ to about ‘y’”.

Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. It will be further understoodthat terms, such as those defined in commonly used dictionaries, shouldbe interpreted as having a meaning that is consistent with their meaningin the context of the specification and relevant art and should not beinterpreted in an idealized or overly formal sense unless expresslydefined herein.

The articles “a” and “an,” as used herein, mean one or more when appliedto any feature in embodiments of the present invention described in thespecification and claims. The use of “a” and “an” does not limit themeaning to a single feature unless such a limit is specifically stated.The article “the” preceding singular or plural nouns or noun phrasesdenotes a particular specified feature or particular specified featuresand may have a singular or plural connotation depending upon the contextin which it is used.

The term “small molecule,” as used herein, refers to a molecule having amolecular weight of about 1000 g/mol, about 800 g/mol, about 600 g/mol,about 500 g/mol, about 400 g/mol, about 350 g/mol, about 300 g/mol,about 250 g/mol, about 200 g/mol, or less.

The term “thermoset polymer,” as used herein, refers to polymerscharacterized by rigid, three-dimensional network structures with highcross-link densities and high molecular weight. Thermoset polymers, whenset, generally stay in the shape when heated until they decompose.Thermoset polymers are insoluble in most common solvents, except forunder the specific conditions and using the specific solvents providedfor herein.

Thermoset Polymers and Composites Thereof

Various methods are provided herein for processing thermoset polymersand composites containing thermoset polymers. The thermoset polymersdescribed herein include an exchangeable covalent bond capable of a bondexchange reaction with a suitable small molecule solvent. For example,where the thermoset polymer contains a plurality of ester bonds, theester bonds can undergo bond exchange reactions with small moleculealcohols. See, for example FIG. 1B and FIGS. 2A-2C. In addition to theester-containing polymers, the bond exchange can be readily extended toother types of thermoset. For example, epoxy thermosets with dynamic C—Sbonds can undergo bond exchange with a thiol-containing solvent (e.g.2-mercaptoethanol solvent). Polyimide thermosets with dynamic C—N bondscan undergo bond exchange with an amine solvent.

In various aspects, the thermoset polymers include a polymer matrixhaving a plurality of exchangeable covalent bonds. The exchangeablecovalent bonds can be ester bonds, sulfide bonds, imide bonds, or othercovalent bonds capable of undergoing bond exchange with a small moleculesolvent at temperatures of about 300° C., about 275° C., about 250° C.,about 225° C., about 200° C., or less.

In some aspects, the exchangeable covalent bond is an ester capable ofundergoing bond exchange reaction with a small molecule alcohol at atemperature of about 300° C., about 275° C., about 250° C., about 225°C., about 200° C., or less. For example, the thermoset polymer matrixcan include an anhydride cured epoxy, an unsaturated polyester, or acombination thereof.

In some aspects, the exchangeable covalent bond is a sulfide capable ofundergoing bond exchange reaction with a small molecule thiol at atemperature of about 300° C., about 275° C., about 250° C., about 225°C., about 200° C., or less. For example, the thermoset polymer matrixcan include a dithiodianiline cured epoxy, a disulfide based epoxy, or acombination thereof.

In some aspects, the exchangeable covalent bond is an imide capable ofundergoing bond exchange reaction with a small molecule amine at atemperature of about 300° C., about 275° C., about 250° C., about 225°C., about 200° C., or less. For example, the thermoset polymer matrixcan include a polyimine crosslinked by tris(2-aminoethyl)amine with aspacer monomer of diethylene triamine, thylene diamine, or a combinationthereof.

The bond exchange reaction can be facilitated by the presence of theappropriate catalyst. For example, where the exchangeable covalent bondis an ester and the small molecule solvent is an alcohol, the catalystcan be an appropriate transesterification catalyst. The catalyst can bepresent in any amount suitable to catalyze the bond exchange reactionbetween the exchangeable covalent bonds and the small molecule solvent.In some aspects, the catalysts is present in an amount from about 1mol-% to about 15 mol-%, 2 mol-% to about 15 mol-%, about 2 mol-% toabout 12 mol-%, about 2 mol-% to about 10 mol-%, or about 3 mol-% toabout 10 mol-%.

The catalyst can be a transesterification catalyst, for example a leadoxide, a lead sulfide, a lead hydroxide, a plumbite, a plumbate, a leadcarbonate, a copper compound, a silver compound, a gold compound, a zinccompound, a cadmium compound, an iron compound, a cobalt compound, asalt thereof, or a combination thereof. The transesterification catalystcan include a metal-ligand complex, wherein the metal-ligand complexincludes a metal such as Cu, Li, Zn, Cd, Fe, or Co; and wherein themetal-ligand complex includes one or more ligands such as acetylacetone,a halide, or a combination thereof.

In some aspects, the thermoset polymer matrix includes a plurality ofester bonds and the small molecule solvent is a small molecule alcohol.The bond exchange reaction activation temperature for the plurality ofester bonds can be about 110° C., about 120° C., about 130° C., about140° C., or higher. For example, the bond exchange reaction activationtemperature for the plurality of ester bonds can be about 110° C. to180° C., about 110° C. to 150° C., about 120° C. to 150° C., about 120°C. to 140° C., or about 110° C. to 130° C.

In some aspects, the thermoset polymer matrix includes a plurality ofsulfide bonds and the small molecule solvent is a small molecule thiol.The bond exchange reaction activation temperature for the plurality ofsulfide bonds can be about 80° C., about 100° C., or higher. Forexample, the bond exchange reaction activation temperature for theplurality of sulfide bonds can be about 80° C. to 110° C. or about 100°C. to 130° C.

In some aspects, the thermoset polymer matrix includes a plurality ofimide bonds and the small molecule solvent is a small molecule amine.The bond exchange reaction activation temperature for the plurality ofimide bonds can be about 40° C., or about 90° C., or higher. Forexample, the bond exchange reaction activation temperature for theplurality of imide bonds can be about 40° C. to 90° C., or about 90° C.to 140° C.

The methods described herein can be applied to thermoset polymers aswell as to composites containing thermoset polymers. A particularlypreferred composite is a carbon fiber reinforced polymer. Carbon fiberreinforced polymers can be difficult to process, and especiallydifficult to recycle without decreasing the mechanical integrity of thecarbon fibers (the most expensive components). The methods are, however,not limited to carbon fiber composites. For example, the composite caninclude a thermoset polymer matrix and a reinforcing material dispersedwithin the thermoset polymer matrix.

Suitable reinforcing materials for use in composites can include, butare certainly not limited to, a glass fiber, a carbon fiber, an aramidfiber, a boron fiber, a graphite, or a combination thereof. Thereinforcing material can have a variety of structures, for example acontinuous fiber, a cloth, a fabric, a yarn, or a tape.

The reinforcing material can have a thermal decomposition temperaturethat is about 500° C., about 450° C., about 400° C., about 350° C., orless. The thermal decomposition is not limited to temperatures where thereinforcing material is completely decomposed, but rather totemperatures where the mechanical strength of the reinforcing materialis damaged, e.g. damaged by at least 1%, at least 5%, or at least 10%.

Small Molecule Solvents

The methods can be used with a variety of small molecule solventssuitable for bond exchange reaction with the exchangeable covalent bondsin the thermoset polymer. For example, the small molecule solvent can bean alcohol, a thiol, an amine, or other suitable small molecule solventcapable of bond exchange with the exchangeable covalent bonds in thethermoset polymer. The small molecule solvent will generally have aboiling point that is greater than the bond exchange reaction (BER)activation temperature for the plurality of exchangeable covalent bondsin the thermoset polymer.

In some aspects, the small molecule solvent is a small molecule alcohol.The small molecule alcohol can have a boiling point of about 140° C. to200° C., about 140° C. to 180° C., about 160° C. to 180° C., about 140°C. to 160° C., about 160° C. to 200° C., or about 180° C. to 200° C. Thesmall molecule alcohol can be a polyol, e.g. a diol, a triol, or otherpolyol having four or more alcohol groups. The small molecule alcoholcan be ethylene glycol, propylene glycol, 1,3-butanediol, diethyleneglycol, 2-ethyl-hexanol, or benzyl alcohol. The small molecule alcoholcan have a molecular weight of about 20 g/mol to 200 g/mol, about 50g/mol to 200 g/mol, about 50 g/mol to 150 g/mol, about 80 g/mol to 200g/mol, about 80 g/mol to 150 g/mol, or about 50 g/mol to 100 g/mol. Theboiling point of the small molecule alcohol can be about 350° C., about300° C., about 275° C., about 250° C., about 225° C., about 200° C., orless.

In some aspects, the small molecule solvent is a small molecule thiol.The small molecule thiol can have a boiling point of about 90° C. to180° C., about 90° C. to 160° C., about 90° C. to 130° C., about 90° C.to 110° C., about 110° C. to 180° C., or about 110° C. to 160° C. Thesmall molecule thiol can include 1 or more thiol groups. The smallmolecule thiol can be 2-Mercaptoethanol, Dithiothreitol,3-Mercaptopropane-1,2-diol, Butanethiol, or 1,3-Propanedithiol. Thesmall molecule thiol can have a molecular weight of about 20 g/mol to200 g/mol, about 50 g/mol to 200 g/mol, about 50 g/mol to 150 g/mol,about 80 g/mol to 200 g/mol, about 80 g/mol to 150 g/mol, or about 50g/mol to 100 g/mol. The boiling point of the small molecule thiol can beabout 350° C., about 300° C., about 275° C., about 250° C., about 225°C., about 200° C., or less.

In some aspects, the small molecule solvent is a small molecule amine.The small molecule amine can have a boiling point of about 60° C. to210° C., about 60° C. to 170° C., about 60° C. to 130° C., about 100° C.to 210° C., about 100° C. to 170° C., or about 100° C. to 130° C. Thesmall molecule amine can include 1, 2, 3, or more amine groups. Thesmall molecule amine can be Hexylamine, 2-Ethyl-1-hexylamine,2-Amino-5-methylhexane, Ethylenediamine, or Diethylenetriamine. Thesmall molecule amine can have a molecular weight of about 20 g/mol to200 g/mol, about 50 g/mol to 200 g/mol, about 50 g/mol to 150 g/mol,about 80 g/mol to 200 g/mol, about 80 g/mol to 150 g/mol, about 100g/mol to 150 g/mol, or about 100 g/mol to 130 g/mol. The boiling pointof the small molecule amine can be about 350° C., about 300° C., about275° C., about 250° C., about 225° C., about 200° C., or less.

Methods of Recycling Thermoset Polymers and Composites Thereof

Methods are provided for recycling thermoset polymer and compositecontaining thermoset polymers. The methods can include (A) washing thethermoset polymer or a composite thereof in a small molecule solvent inthe presence of a catalyst at a first elevated temperature for a firstperiod of time sufficient to dissolve the thermoset polymer matrix; and(B) evaporating the small molecule solvent at a second elevatedtemperature for a second period of time to re-polymerize the thermosetpolymer matrix. The thermoset polymer matrix should contain a pluralityof exchangeable covalent bonds capable of bond exchange with the smallmolecule solvent in the presence of the catalyst and at the firstelevated temperature. The small molecule solvent can have a boilingpoint above the bond exchange reaction (BER) activation temperature forthe plurality of exchangeable covalent bonds.

In some aspects, the thermoset polymer matrix includes a plurality ofester bonds, and the method includes (A) washing the thermoset polymeror a composite thereof in a small molecule alcohol in the presence of acatalyst at a first elevated temperature for a first period of timesufficient to dissolve the thermoset polymer matrix; and (B) evaporatingthe small molecule alcohol at a second elevated temperature for a secondperiod of time to re-polymerize the thermoset polymer matrix. The smallmolecule alcohol can have a boiling point above the bond exchangereaction (BER) activation temperature for the plurality of ester bonds.

In some aspects, the thermoset polymer matrix includes a reinforcingmaterial dispersed within the thermoset polymer matrix, e.g. thematerial is a composite containing a thermoset polymer. In some aspects,the composite is a carbon fiber reinforced polymer. The methods caninclude recycling or recovering one or both of the carbon fiber and thethermoset polymer from the carbon fiber reinforced polymer.

Where the carbon fiber reinforced polymer includes (i) a thermosetpolymer matrix including a plurality of exchangeable covalent bonds and(ii) a carbon fiber dispersed within the thermoset polymer matrix; themethod can include (A) washing the carbon fiber reinforced polymer in asmall molecule solvent in the presence of a catalyst at a first elevatedtemperature for a first period of time sufficient to dissolve thethermoset polymer matrix and reclaim the carbon fiber; wherein the smallmolecule solvent has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of exchangeable covalentbonds; and wherein the boiling point of the small molecule solvent isbelow a thermal decomposition temperature of the carbon fiber.

Where the carbon fiber reinforced polymer includes (i) a thermosetpolymer matrix including a plurality of ester bonds and (ii) a carbonfiber dispersed within the thermoset polymer matrix; the method caninclude (A) washing the carbon fiber reinforced polymer in a smallmolecule alcohol in the presence of a catalyst at a first elevatedtemperature for a first period of time sufficient to dissolve thethermoset polymer matrix and reclaim the carbon fiber; wherein the smallmolecule alcohol has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of ester bonds; andwherein the boiling point of the small molecule alcohol is below athermal decomposition temperature of the carbon fiber.

The methods can also include evaporating the small molecule solvent at asecond elevated temperature for a second period of time to re-polymerizethe thermoset polymer matrix. This can be done either (i) to reform thecomposite by re-polymerizing the thermoset polymer matrix with thereinforcing material still in place or (ii) to recover the thermosetpolymer separately from the reinforcing material by separating thereinforcing material from the solvent prior to the re-polymerization ofthe thermoset polymer matrix. For example, by recombining the smallmolecule alcohol and the reclaimed carbon fiber prior to the evaporatingstep, the re-polymerizing of the thermoset polymer matrix can produce arecycled carbon fiber reinforced polymer including (i) the thermosetpolymer matrix with the plurality of ester bonds and (ii) the reclaimedcarbon fiber dispersed within the thermoset polymer matrix.

In some aspects, or both of the first elevated temperature and thesecond elevated temperature can be between the bond exchange reactionactivation temperature and the decomposition temperature of thereinforcing material. In some aspects, one or both of the first elevatedtemperature and the second elevated temperature can be between the bondexchange reaction activation temperature and just above the boilingpoint of the small molecule solvent. In some aspects, one or both of thefirst elevated temperature and the second elevated temperature can beindependently about 120° C. to 250° C., about 120° C. to 220° C., about120° C. to 200° C., about 140° C. to 220° C., about 140° C. to 200° C.,about 160° C. to 220° C., about 160° C. to 200° C., about 150° C. to200° C., about 170° C. to 210° C., or about 180° C.

In some aspects, the first period of time is long enough to dissolve allor part of the thermoset polymer matrix. In some aspects, the secondperiod of time is long enough to evaporate at least about 90%, about95%, about 98%, about 99%, or more of the small molecule solvent. Insome aspects, one or both of the first period of time and the secondperiod of time are independently about 1 hours to 10 hours, about 1hours to 8 hours, about 1 hours to 6 hours, about 2 hours to 6 hours,about 2 hours to 8 hours, or about 2 hours to 10 hours.

The recycled materials can retain the mechanical integrity of thematerials prior to recycling. For example, where the composite isreformed the recycled composite can retain an elastic modulus that iswithin 95%, with 97%, within 98%, within 99%, or within 99.5% of theelastic modulus of the composite prior to the recycling and whenmeasured under the same conditions. In some aspects, the reinforcingmaterial is reclaimed and the reinforcing material has an elasticmodulus that is within 95%, with 97%, within 98%, within 99%, or within99.5% of the elastic modulus of the reinforcing material prior to therecycling and when measured under the same conditions. In some aspects,the reclaimed material is the carbon fiber from a carbon fiberreinforced polymer and the reclaimed carbon fiber can retain an elasticmodulus that is within 95%, with 97%, within 98%, within 99%, or within99.5% of the elastic modulus of the carbon fiber prior to the recyclingand when measured under the same conditions. In some aspects, theelastic modulus is measured by uniaxial tension tests performed on auniversal materials testing machine by following ASTM D638 standard.

The recycled materials can retain the strength of the materials prior torecycling. For example, where the composite is reformed the recycledcomposite can retain a tensile strength that is within 95%, with 97%,within 98%, within 99%, or within 99.5% of the tensile strength of thecomposite prior to the recycling and when measured under the sameconditions. In some aspects, the reinforcing material is reclaimed andthe reinforcing material has a tensile strength that is within 95%, with97%, within 98%, within 99%, or within 99.5% of the tensile strength ofthe reinforcing material prior to the recycling and when measured underthe same conditions. In some aspects, the reclaimed material is thecarbon fiber from a carbon fiber reinforced polymer and the reclaimedcarbon fiber can retain a tensile strength that is within 95%, with 97%,within 98%, within 99%, or within 99.5% of the tensile strength of thecarbon fiber prior to the recycling and when measured under the sameconditions. In some aspects, the tensile strength is measured byuniaxial tension tests performed on a universal materials testingmachine by following ASTM D638 standard. e

Methods of Repairing Thermoset Polymers and Composites Thereof

In various aspects, methods are provided for repairing thermosetpolymers and composites containing thermoset polymers. For example, themethods of repair can be applied where the surface of a thermosetpolymer or a composite thereof contains one or more imperfections, e.g.scratches, dents, etc.

In some aspects, where the thermoset polymer matrix includes a pluralityof exchangeable covalent bonds, the methods can include (A) applying apowder of the thermoset polymer to the surface of the thermoset polymeror composite thereof, (B) contacting the surface of the thermosetpolymer or composite thereof and the powder with a small moleculesolvent in the presence of a catalyst at a first elevated temperaturefor a first period of time sufficient to dissolve the powder and atleast a portion of the thermoset polymer matrix at the surface of thethermoset polymer or composite thereof, and (C) evaporating the smallmolecule solvent at a second elevated temperature for a second period oftime sufficient to re-polymerize the thermoset polymer matrixincorporating the thermoset polymer from the powder to repair theimperfection.

In some aspects the thermoset polymer matrix includes a plurality ofester bonds and the small molecule solvent is a small molecule alcohol.The methods can include (A) applying a powder of the thermoset polymerto the surface of the thermoset polymer or composite thereof, (B)contacting the surface of the thermoset polymer or composite thereof andthe powder with a small molecule alcohol in the presence of a catalystat a first elevated temperature for a first period of time sufficient todissolve the powder and at least a portion of the thermoset polymermatrix at the surface of the thermoset polymer or composite thereof, and(C) evaporating the small molecule alcohol at a second elevatedtemperature for a second period of time sufficient to re-polymerize thethermoset polymer matrix incorporating the thermoset polymer from thepowder to repair the imperfection.

In some aspects, or both of the first elevated temperature and thesecond elevated temperature can be between the bond exchange reactionactivation temperature and the decomposition temperature of thereinforcing material. In some aspects, one or both of the first elevatedtemperature and the second elevated temperature can be between the bondexchange reaction activation temperature and just above the boilingpoint of the small molecule solvent. In some aspects, one or both of thefirst elevated temperature and the second elevated temperature can beindependently about 120° C. to 250° C., about 120° C. to 220° C., about120° C. to 200° C., about 140° C. to 220° C., about 140° C. to 200° C.,about 160° C. to 220° C., about 160° C. to 200° C., about 150° C. to200° C., about 170° C. to 210° C., or about 180° C.

In some aspects, the first period of time is long enough to dissolve allor part of the thermoset polymer matrix. In some aspects, the secondperiod of time is long enough to evaporate at least about 90%, about95%, about 98%, about 99%, or more of the small molecule solvent. Insome aspects, one or both of the first period of time and the secondperiod of time are independently about 1 hours to 10 hours, about 1hours to 8 hours, about 1 hours to 6 hours, about 2 hours to 6 hours,about 2 hours to 8 hours, or about 2 hours to 10 hours.

Methods of Welding Thermoset Polymers and Composites Thereof

In various aspects, methods are provided for chemically weldingthermoset polymers and composites containing thermoset polymers. Forexample, methods are provided for welding a first surface to a secondsurface, wherein both the first surface and the second surface include athermoset polymer matrix having a plurality of exchangeable covalentbonds. The methods can include (A) contacting the first surface and thesecond surface with a small molecule solvent in the presence of acatalyst at a first elevated temperature for a first period of time; (B)contacting the first surface and the second surface to form aninterface; and (C) evaporating the small molecule solvent to polymerizethe thermoset polymer matrix at the interface.

In some aspects, the first surface and the second surface include aplurality of ester bonds and the small molecule solvent is a smallmolecule alcohol. The methods can include (A) contacting the firstsurface and the second surface with a small molecule alcohol in thepresence of a catalyst at a first elevated temperature for a firstperiod of time; (B) contacting the first surface and the second surfaceto form an interface; and (C) evaporating the small molecule alcohol topolymerize the thermoset polymer matrix at the interface.

The methods can be used to chemically weld a variety of surfaces, forexample to weld two thermoset polymers together, to weld to compositestogether containing a thermoset polymer matrix, or to weld a thermosetpolymer to a composite containing a thermoset polymer.

The methods are capable of forming strong welds. In some aspects, theinterface has an interfacial fracture energy of about 1000 J/m² to 10000J/m², about 1000 J/m² to 5000 J/m², about 1200 J/m² to 5000 J/m², about1500 J/m² to 5000 J/m², about 1500 J/m² to 2500 J/m², about 2000 J/m² to5000 J/m², or about 2000 J/m² to 10000 J/m².

EXAMPLES

Now having described the embodiments of the present disclosure, ingeneral, the following Examples describe some additional embodiments ofthe present disclosure. While embodiments of the present disclosure aredescribed in connection with the following examples and thecorresponding text and figures, there is no intent to limit embodimentsof the present disclosure to this description. On the contrary, theintent is to cover all alternatives, modifications, and equivalentsincluded within the spirit and scope of embodiments of the presentdisclosure.

Example 1: Solvent Assisted Surface Welding and Reprocessing ofMalleable Epoxy Polymers

In this example, ethylene glycol (EG) is used as a solvent to realizepressure-free surface welding and reprocessing of a CAN epoxy system.Here, the epoxy based CAN capable of BER in Montarnal et al (Montarnal,D., et al., Science, 2011. 334(6058): p. 965-968) is adopted and EGsolvent is used, as it has a relatively high boiling point. When themixture of EG solvent and the CAN is heated to the BER activetemperature, the hydroxyl group in the EG participates in exchangereactions with the ester group in the networks; because EG is asmall-molecule solvent, this results in breaking the long polymer chainsat the ester group on the backbone. After dissolving the epoxy, the EGis allowed to evaporate, leaving the dissolved epoxy to reform into asolid. Surface welding can be realized when such dissolution andreformation events occur on the material interfaces. These events arestrongly dependent on the molecule diffusion and the rate of BERs, whichis further determined by temperature and catalyst concentration. A goodcontrol on the diffusion time of alcohol molecules will onlydepolymerize the skin layer of CAN (due to its surface erosion type ofdegradation), which leads to a tacky surface that serve as a glue forpressure-free surface welding. This welding method is also extended tofacilitate polymer reprocessing from the powder state. The approachpromotes the efficiency of recycling thermosets and broadens theirutilization in practical situations.

Materials and Method

Materials and Synthesis

The epoxy based CAN was prepared following the method used by Leiblerand coworkers (Capelot, M., et al., Journal of the American ChemicalSociety, 2012. 134(18): p. 7664-7667; Capelot, M., et al., ACS MacroLetters, 2012. 1(7): p. 789-792.; Montarnal, D., et al. Science, 2011.334(6058): p. 965-968). It was synthesized by using commerciallyavailable materials, including a catalyst, monomers and crosslinkers:metal catalyst Zn(Ac)₂ (Sigma Aldrich, St. Louis, Mo., USA), diglycidylether of bisphenol A (DGEBA, Sigma Aldrich), and fatty acids Pripol 1040(Uniqema Inc., Paterson, N.J., USA). For the alcohol solvent, anhydrousethylene glycol (Sigma Aldrich) with purity of 99.8% was selected due toits high boiling point (197.3° C., above the BER activationtemperature).

FIG. 1A illustrates the main chemical reaction during the synthesis ofCANs. To prepare the epoxy, the catalyst (with different concentrations,0 mol %, 1 mol %, 3 mol %, 5 mol % and 7 mol %, respectively) were mixedwith fatty acids in a round-bottom flask and heated under vacuum. Thetemperature was gradually increased from 100° C. to 180° C., and thenwas held at 180° C. until no gas bubble was observed and catalystparticles were fully solubilized (1 h). DGEBA in solid state was heatedto 130° C. until totally melted, and then was poured into the fatty acidmixture in open air. The new mixture was manually stirred untilhomogeneous and translucent (stoichiometry between COOH and epoxy groupis 1:1). After this, the mixture was placed in vacuum to remove thebubbles, and transferred to an oven for 6 h at 130° C. The CAN epoxy iscapable of BER via transesterification reaction (FIG. 1B) at hightemperature.

Dissolution and Repolymerization

EG solvent is adopted to achieve pressure-free recycling of CAN epoxydue to a relatively higher reaction rate. FIGS. 2A-2C illustrate thedissolution and repolymerization mechanisms of CAN epoxy. In a regularCAN epoxy system, the hydroxyl groups in the ring-opened epoxyparticipate in the transesterification reactions. When EG is added, thehydroxyl groups in EG solvent can also participate transesterificationreactions with ester groups in the polymer network; since EG moleculesare small and are not linked to any long chains, they effectively breakthe long chain polymer into small sections, as shown in FIG. 2A. Itshould be noted that reactions where an EG molecule is regenerated, asshown in FIG. 2B, can also occur; but when EG is in excess amount, thereaction in FIG. 2A dominates. Therefore, as the reactions go on, theepoxy network can be gradually dissolved when sufficient EG solvent isprovided. However, this will change if the reactions are conducted in anenvironment where the EG solvent tends to evaporate. There, the EGsolvent will leave the solution, the reaction shown in FIG. 2B willdominate, resulting in repolymerization of the dissolved thermoset.Because the boiling point of EG is 197.3° C., a moderate temperature of180° C.-200° C. was used and an open-air environment for the epoxyrepolymerization, as it will guarantee the reaction and the evaporationof excessive EG solvent. FIG. 2C schematically shows the dissolution andrepolymerization mechanisms in the network, where EG molecules firstpenetrate into CANs, driven by the difference of EG contents between theinside and outside of the network (I). The epoxy thermoset subsequentlybreaks into short chain segments due to the EG-assistedtransesterification (II). After dissolution, the broken chain segmentsstart to be reconnected, as the EG evaporates at a relatively highertemperature in an open-air environment (III), and eventually, the epoxythermoset is reformed (IV).

To measure the rate of dissolution and repolymerization, epoxy sampleswith different amounts of catalyst were cut into the same dimensions (12mm×10 mm×5 mm). They were then immersed in different amounts of EGsolvent in glass bottles, which were sealed and transferred into an ovenwith elevated temperatures (140° C., 180° C., and 220° C.,respectively). Following the approach by Metters et al. (Metters, A. T.,et al. Polymer, 2000. 41(11): p. 3993-4004), at intervals, the solidsamples were taken out of the EG solvent, cleaned and weighed to monitorthe residual mass as a function of heating time. The full dissolution ofthe epoxy was marked by the complete disappearance of the solid in theEG solvent. To repolymerize the epoxy, the bottle was left open to letEG naturally evaporate at high temperature. Normally, when the dissolvedEG-epoxy solution was exposed to hot air, new epoxy film was formed andsuspended on the top surface of the solution. Next, the film was takenout and wiped on the surface to measure the mass of the repolymerizedepoxy every hour. We note that this approach may not present asufficiently accurate method to measure the degree of polymerization;but it does offer visible and useful information about thesolidification process of the epoxy. Besides, since the repolymerizationis performed at high temperature for a long period, one should considervacuum condition to avoid any significant oxidization.

Thermomechanical Characterization Tests

Two standard polymer tests were used to examine the thermomechanicalproperties (the glass transition and temperature dependent stressrelaxation behaviors) of the repolymerized epoxy. The glass transitionbehaviors of the prepared and regenerated epoxy thermoset were tested bydynamic mechanical analysis (DMA). A strip sample, with the dimension of10.0 mm×5.0 mm×1.5 mm was tested by a DMA tester (Model Q800, TAInstruments, New Castle, Del., USA) on the tensile mode. The stripsample was first heated to 100° C. and thermally equilibrated for ˜20min, and then a preload of 0.001N was applied. During the experiment,the strain was oscillated at a frequency of 1 Hz with a peak-to-peakamplitude of 0.1%. The temperature was decreased from 100° C. to −50° C.at a rate of 1° C./min. Next, the temperature was held at −50° C. for 30min and then increased to 100° C. at the same rate. This procedure wasrepeated multiple times, and the data from the last cooling step wasreported. For the stress relaxation tests of regenerated CANs, a polymersample with the same dimensions mentioned above was first preloaded by0.001N force to maintain straightness. After reaching the testingtemperature (120° C., 140° C., 160° C., 180° C. and 200° C. separately),it was allowed ˜20 min to reach thermal equilibrium. The specimen wasstretched by 1% on the DMA tester, and the deformation was maintainedduring the following tests. The decrease of stress was recorded, and thestress relaxation modulus was calculated.

Pressure-Free Surface Welding Effect

We demonstrated the pressure-free surface welding effect in the CANepoxy by using EG solvent. In the first group of experiments (FIG. 3A),prepared epoxy samples with different amounts of catalysts were cut intostrips with the same dimensions (20 mm×4 mm×2 mm). These samples werethen placed in a glass bottle with excess amounts of EG at thetemperature of 180° C. for 30 min. As will be discussed later, such ashort heating time would only dissolve a thin skin layer of CAN tocreate a sticky surface. After that, the two epoxy strips were attachedtogether, and transferred to a high-temperature oven of 180° C. fordifferent heating durations. It should be pointed out that the selectedtemperature was 180° C. for all of the following welding tests andpowder reprocessing tests; this is high enough to permit the dynamicnature of CANs, as well as offer an acceptable EG evaporation rateduring the repolymerization. Since this method requires immersing thesample in the EG solution for a certain amount of time, it is referredto as a pre-treatment method herein.

In the second group of experiments (FIG. 3B), a small amount of CANepoxy was dissolved in EG solvent. The dissolved polymer solution, whichwill be called EG-epoxy glue in the following, was then used to glue twopieces of pristine strip without pre-soaking in the EG solvent. Thewelding condition was the same as that in the first group ofexperiments. This method is referred to as the EG-epoxy glue methodherein.

The interfacial energy of the welded samples was quantitativelycharacterized by T-peeling tests (D1876-08(2015)e1, ASTM International)over different catalyst concentrations. This method was used previouslyfor studying the healing efficiency of micro-capsuled, catalysttriggered self-healing epoxy. In the tests, an open region was cut atthe end of the welded sample. After the pressure-free welding process,each end of the welded sample was loaded into two opposing grips of theDMA tester at room temperature (23° C.). During the peeling test, thegrips were separated at a constant rate of speed (5%/min) for allsamples. Next, the interfacial energy of the repaired samples wascalculated using the measured force-displacement curves.

The EG-assisted welding was also applied to heal surface damage. FIG. 3Cshows a schematic illustration for the procedure of damage repair. Inthis (the third) group of experiments, surface damage was prepared byscratching the surface of CANs by using a needle. The EG-epoxy glue wasadded to the damaged surface, after which the sample was placed in a180° C. chamber for 3 h. As will be discussed in the following, theepoxy glue will flow on the CAN surface at high temperature and coverthe damaged area, which eventually leads to a smoothly healed surface.

Pressure-Free Reprocessing Ability

The EG-assisted welding method was also used to reprocess the CAN epoxyfrom the powder state. FIG. 4 illustrates the experimental procedure. Afresh CAN epoxy was first ground into powder using a manual grinder. Thetypical size of the polymer powder is from ˜1.30 mm to ˜1.65 mm. Second,3 g EG was added into 0.3 g ground powders followed by manual stirringfor 3 min. Third, the homogeneous mixture was transferred intoflat-bottomed glassware and heated at 180° C. for 4 h.

To evaluate the mechanical properties of the reprocessed sample, the DMAtester was used in tensile mode to test the mechanical behaviors at roomtemperature (25° C.). The loading rate was chosen to be a small value(5% per min for all tests) to minimize viscoelastic effects. The sampleswere trimmed uniformly to the size of 12 mm×4 mm×1 mm for testing. Foreach tension test, at least four samples were tested and the averagevalues with error bars were reported.

Fourier Transform Infrared Spectroscopy Measurement

To determine the actual composition and the structure of the networksafter a dissolution/re-crosslinking cycle, Fourier transform infraredspectroscopy (FTIR) tests were conducted to monitor and characterize theconversion of functional groups (esters, hydroxyls, and epoxy COCgroups) in both EG-assisted dissolution and repolymerization process.

Five samples were prepared before the FTIR measurements: 1) fresh CANs,2) pre-dissolved CANs that was dissolved in EG solvent at 180° C. for 60min, 3) fully dissolved CANs after soaking in EG at 180° C. for 240 min,4) pre-crosslinked CANs that was repolymerized at 180° C. for 60 minfrom a fully dissolved epoxy solution. 5) fully recycled CANs that wasrepolymerized at 180° C. for 180 min. The FTIR measurements wereperformed at the room temperature by using Thermo Scientific NicoletiS50 FT-IR spectrometer fitted with an ATR cell. Tests were carried outfor all these 5 samples respectively. The spectrum of each sample is theaveraged result from 32 scans collected from 550⁻¹ to 4000 cm⁻¹ at 1cm⁻¹ resolution.

Results

Time and Temperature Dependent Dissolution and Repolymerization of CANEpoxy

The dissolution of CAN epoxy immersed in high-temperature EG solvent wasfirst characterized. FIG. 5A presents the evolution of appearance andsize of a sample after being heated to 180° C. in EG solvent fordifferent time periods. Apparently, the dissolution of CAN epoxy is asurface erosion type. The color of the fully dissolved CAN epoxy-EGsolutions with various catalyst concentrations varied a little with theincrease of catalyst content. The viscosity of all the EG dissolutionsis relatively low, which assists their spreading on the material'ssurface when they are used for surface welding and surface damagerepair. FIG. 5B shows the normalized weight of the epoxy sample as afunction of heating time with different amounts of EG solvent. For FIG.5C, the catalyst concentration is 5 mol % and the heating temperature is180° C. When the EG content is greater than 0.4 g, the dissolution ratein each case maintains a constant around 0.01 g/min. It should be notedthat since the dissolution is of the surface erosion type, this numbershould depend on the surface area or volume. Considering the initialsample dimension is 12 mm×110 mm×5 mm, the dissolving rate is ˜2.2×10⁻⁵g mm⁻²/min.

FIGS. 5C-5D show the change in the normalized weight of the epoxy sampleas a function of catalyst concentration and heating temperature,respectively. In FIG. 5C, it is seen that when no BER (or no catalyst)is involved in the epoxy network, no dissolution was observed in thesamples even after immersion in the solvent for 4 hours. The epoxyabsorbs the EG solvent, and the final weight increase is ˜4 wt %. Thisobservation confirms that the transesterification type BERs are thereason for the dissolution of CAN epoxy. In addition, FIG. 5C also showsthat the initiation of dissolution is delayed by decreasing the amountof catalyst. However, it is interesting to find that the subsequentdissolution rate is about the same at 0.01 g/min. In contrast, FIG. 5Didentifies the changes in dissolution rate due to the different heatingtemperatures. For example, when the temperature is increased from 140°C. to 220° C., the dissolution rate increases from 0.0032 g/min to˜0.025 g/min. We conjecture that this temperature response can beexplained according to the three processes that determine thedissolution rate: the diffusion of EG molecules into the polymernetwork, the breaking and reforming of polymer chains due to the BERs,and the diffusion of broken chain segments into the solvent. In FIG. 5C,for example, a higher concentration of catalyst leads to a higher rateof BERs, so the epoxy sample with 7 mol % catalyst loses its weightfirst. However, the subsequent dissolution is limited by the rate atwhich polymer chains diffuse away from the polymer-solvent interface.Since this diffusion mechanism mainly depends on the temperature, thedissolution rate in each case of In FIG. 5C, is almost the same afterthe starting point. On the other hand, when the temperature isincreased, all the aforementioned three processes speed up, whichconsequently leads to a higher rate of dissolution of the CAN epoxysample (see FIG. 5D).

As discussed above, the transesterification-type BER allowsrepolymerization of CAN epoxy when EG is evaporated. FIG. 6 shows therate of repolymerization by measuring the accumulated solid mass duringthe epoxy solidification. It is seen that at the temperature of 200° C.,the mass of repolymerized epoxy is in an almost linear relation with theheating time, and the repolymerization rate is ˜0.23 g/h (0.0038 g/min).In the experiments, we also found that because the density of dissolvedepoxy is lower than that of EG solvent, it suspends in the top layer ofthe solution. Such a phase separation between dissolved epoxy and EG isshown in bottom inset of FIG. 6. For a dissolved polymer solution withfinite depth, the repolymerization always starts on the solution-airinterface where EG molecules first evaporate out of the mixture. Thisled to a solidified film on the surface, which acts as a barrier forfurther solvent evaporation. The suspended epoxy thin film will suppressthe EG evaporation, and consequently the repolymerization of the epoxybeneath it. So the repolymerized epoxy may initially possesshierarchical structure with different crosslinking degree in thethickness direction. For current study, we give a sufficiently longrepolymerization time so the BERs eventually lead to a uniform network.

Fourier transform infrared spectroscopy (FTIR) tests were conducted onepoxy samples in both dissolution and repolymerization steps. FIGS.14A-14B trace the conversion of esters, epoxy COC and hydroxyls. Anotable characteristic peak of esters is observed at 1735 cm⁻¹ in FIG.14A which remarkably decreases in the dissolution process and thencompletely recovers to the original level during repolymerization. Onthe other hand, the corresponding hydroxyl groups in FIG. 14B increasesas the progressive dissolution proceeds, and then drops to the samelevel as that in fresh CANs after being heated at 180° C. for 180 min.This demonstrates that there is no remnant EG molecules left in theepoxy network, and the dissolved polymer solution is fully polymerized.On the other hand, the FIG. 14A also shows a relatively stable trend ofthe epoxy COC. It means that this group hardly participates in theEG-assisted transesterification type BERs. The conversions only occurbetween hydroxyls and esters during the EG-assisted transesterificationtype BERs. The results confirm that both ester and hydroxyl groups inrecycled epoxy return to the same level of those in fresh epoxy, so allthe EG solvent is evaporated and the dissolved polymer solution is fullypolymerized into epoxy thin film. The results also imply that a finercontrol of exposure area and mechanical stirring will accelerate therate of repolymerization. Consequently, mechanical stirring will be usedin the next section to regenerate homogeneous CANs.

The aforementioned dissolution and repolymerization cycle was alsotested by using a mono-alcohol 2-ethyl hexanol (2E1H), and the testingresults are described below. It is seen that the epoxy thermosets canstill be fully dissolved and repolymerized into a new generation epoxysample. FTIR tests confirm that both ester and hydroxyl groups in therecycled epoxy return to the same level as that of fresh epoxy. However,it shows that that the dissolution and repolymerization rates in 2E1Hare much smaller than that in EG solvent when the same temperature isgiven. This is because that each 2E1H molecule only provides onehydroxyl group, which consequently leads to a smaller BER efficiencycompared with that using di-functional alcohol EG.

Thermomachanical Property of the Repolymerized CANs

In this section, we characterize the thermomechanical properties of therepolymerized epoxy with 5 mol % catalyst. The material was obtained byheating 3 g of dissolved polymer solution on a hotplate at 200° C. withmagnetic stirring. After ˜30 min, the mixture was transferred to an ovenat 200° C. for another 2.5 h. The former heating with mechanicalstirring allows the quick evaporation of residual EG in the solution andhomogeneous reformation of a solid epoxy material. FIG. 7A shows thenormalized tan δ and storage modulus of both fresh and repolymerizedepoxy as a function of temperature. It is seen that the storage modulusof the recycled epoxy is larger than that of fresh one, which might beresulted from the oxidation of epoxy during the repolymerization.However, since the modulus change is small, we expect a minimalinfluence of oxidation on the BER kinetics and associatedthermomechanical properties of epoxy thermosets. In real application,oxidation should be addressed and controlled. The temperature at thepeak of tan δ is referred as the glass transition temperature T_(g) ofthe material. The T_(g) of the repolymerized material is 30.1° C., whichis very close to that of fresh epoxy (33.3° C.). FIG. 7B shows is acomparison of normalized stress relaxation curves at differenttemperatures (namely, 120° C., 140° C., 160° C., 180° C. and 200° C.)between the repolymerized and fresh CANs. The results show that theregenerated material has nearly the same stress relaxation behavior asthe fresh epoxy. Epoxy thermosets recycled by using mono-alcohol 2E1H isalso seen to exhibit the same glass transition behavior as freshmaterial (FIG. S2 in the Supplementary Materials). The stress relaxationbehavior of the thermoset can be captured by using a simple exponentialfunction, namely:

$\begin{matrix}{{\frac{\sigma}{\sigma_{0}} = {\exp \lbrack {- \frac{t}{\tau}} \rbrack}},} & (1)\end{matrix}$

with the stress relaxation time:

$\begin{matrix}{\tau = {\frac{1}{k}{{\exp \lbrack \frac{E_{a}}{R( {T + 273.15} )} \rbrack}.}}} & (2)\end{matrix}$

In the above equations, σ₀ is the stress before relaxation, k is akinetic coefficient (k>0), R is the gas constant of 8.314 J/(mol·K), Tis the Celsius temperature, and E_(a) is the activation energy for BERs.In FIG. 7B, after fitting the experimental curves by using exponentialfunctions, we can obtain the stress relaxation times at differenttemperatures, and they are plotted in FIG. 7C. It is seen that therelaxation times of the repolymerized epoxy agree very well with thoseof fresh epoxy. The activation energy E_(a) is ˜70 kJ/mol, which isclose to that of fresh epoxy reported before (68.2 kJ/mol). Based on theabove mentioned similarities of glass transition point and stressrelaxation capability, we can claim that the epoxy thermosets can beeasily and efficiently recycled via EG-assisted transesterification BERswithout pressure.

EG-Assisted Surface Welding of the CAN Epoxy

The interfacial welding ability of the CAN epoxy was investigated. Here,we demonstrate the welding process, where it can be done over anysurfaces rather than exclusively the fresh cut ones, and weldingpressure is not required. Two methods for pressure-free welding arepresented. The first of these is the pre-treatment approach, where theepoxy strips were pre-treated by being soaked in the EG solvent at hightemperature and then attached together for welding.

FIG. 8A shows the results obtained from tests of the pre-treatmentwelding method. First, the prepared CANs were cut into strips withspecific dimensions. The cut surface was observed by an opticalmicroscope (model V5MP, VWR International, Radnor, Pa., USA) (seebottom-left in FIG. 8A), which shows many tiny defects. There are alsosome scratches due to the cutting blade. The sample was then immersed inthe EG solvent at 180° C. for 30 min. Polymer dissolution occurred onthe surface (see bottom-left in FIG. 8A), which shows that theoriginally rough surface (in bottom-left) became smooth. Next, the twostrips were stacked together at room temperature without applyingpressure; because the dissolved surface was tacky, the two strips stucktogether easily. Finally, the two strips were transferred to an ovenmaintaining 180° C. for 3 h. FIG. 8B shows optical images of thecompletely welded sample in the top surface and middle surface. Here,the middle surface was obtained by cutting the welded sample along thegreen lines (shown in FIG. 8B). The top surface image shows the mark ofthe interface; however, the image from the cut in the middle does notshow any sign of an interface, indicating almost perfect welding. Thisis different from our previous work for surface welding, where weldingpressure was necessary to close the voids on the interface, yet therewere still voids and debris (Yu, K., et al., Journal of the Mechanicsand Physics of Solids). In the current work, no pressure is applied, yetan almost perfect welding is achieved. This is mainly for two reasons:first, the pre-treated epoxy surface is smooth enough and is in viscousliquid state, which guarantees a good contact on the interface (see FIG.8A); second, the surface layer of the epoxy sample has been partiallydissolved before welding. The capillary force of the viscous solutioncould further assist the mixing and bonding of the interfacial material.It should be noted that the quality of this welding method depends onthe pre-treatment time of CAN. Over-soaking an epoxy thermoset samplewill lead to excessive surface corrosion, and consequently notabledeformation of sample after welding. A good selection of pre-treatmenttime can maintain both welding strength and welding quality at the sametime

In the second method of pressure-free welding, the epoxy was fullydissolved first, and the dissolved polymer solution was used as glue toweld another two pieces of fresh epoxy sample (shown in FIG. 9A). In ourexperiments, five drops of dissolved epoxy (˜0.02 ml) were carefullycoated onto the surface of two epoxy strips (20 mm×4 mm×2 mm). We alsorubbed the two surfaces against each other to ensure a good spreading ofthe EG-epoxy glue. The EG-epoxy glue makes the two samples easily sticktogether. Finally, the two strips were transferred to an oven at 180° C.for 3 h (no pressure was applied during the welding). To examine theinfluence of catalyst concentration on the welding efficiency, fivegroups of epoxy thermosets with different amounts of catalyst (1 mol %,3 mol %, 5 mol % and 7 mol % respectively) were dissolved and used asglue for the aforementioned EG-epoxy glue welding. It should be notedthat the catalyst concentration in the epoxy samples to be welded wasalways the same as that in the epoxy glue. Optical morphology images ofthe top and middle surfaces of the welded epoxy samples are representedin the bottom figure of FIG. 9B. In contrast to the tests ofpre-treatment welding, an interface can be observed between the weldedepoxy strips (see the red arrows).

To quantify the dependence of welding efficiency on catalystconcentration, the interfacial fracture energy was tested by using theT-peeling test. The inset in FIG. 10A shows the experimental setup, andFIG. 10A shows the typical peeling force vs. displacement curves for thepre-treatment and EG-epoxy glue methods, where the two epoxy strips(with 5 mol % catalyst) were welded at 180° C. for 180 min. It should benoted that although FIG. 10A shows that the EG-epoxy glue methodrequires a higher peeling force, its magnitude actually depends on thegeometrical size and pre-treatment conditions, such as time andtemperature, which deserves more study in the future.

According to the Griffith's energy balance law, when the weldingsubstrate is an elastomeric polymer, the cohesive fracture energy G_(c)can be calculated as:

$\begin{matrix}{{G_{c} = {\frac{2{\overset{\_}{P}( {1 + e} )}}{b} - {E_{0}{he}^{2}}}},{{{with}\mspace{14mu} \overset{\_}{P}} = \frac{\int_{t_{1}}^{t_{2}}{{P(t)}{dt}}}{t_{2} - t_{1}}},} & (3)\end{matrix}$

where P is the average peeling force during the steady propagation ofthe crack (from t₁ to t₂); E₀ is the modulus of the material, h is thesample thickness, b is the width, and e is the elastic strain underforce P.

FIGS. 10B-10C show the effects of catalyst concentration and weldingtime on the fracture energy of pre-treatment and EG-epoxy glue welding,respectively. It is noted that the interfacial energy increases bothwith the catalyst concentration and with the welding time (see FIGS.10B-10C). It is apparent that sufficient reaction time can yield enoughBERs over the interface. The more crosslinked chains that penetrate intothe opposite face of an interface, the higher the fracture energy isobtained. In addition, a higher catalyst concentration leads to a higherBER rate, which also increases the interfacial fracture energy. As shownin the figures, for the pre-treatment welding method, the fractureenergy after welding for 180 min increases from 1600 J/m² to 2300 J/m²when the catalyst concentration is increased from 1 mol % and 5 mol %;for the EG-epoxy glue method, the corresponding fracture energy isincreased from 1200 J/m² to 3200 J/m². FIG. 10C also shows that themaximum fracture energy reaches a value over 4000 J/m² at 7 mol %catalyst heating for 180 min.

As mentioned above, the welding methods are completely pressure-free andare only driven by EG solvent, which are advantageous in many practicalapplications of adaptable thermosets. In FIG. 10D the cohesivelyfracture energy of epoxy samples welded under different pressures(namely: 2 kPa, 40 kPa and 90 kPa) are plotted together with that inpressure-free pre-treatment and EG-epoxy glue welding, where thecatalyst concentration in the epoxy CANs are all 5 mol %. Overall, when40 kPa and 90 kPa is applied, the pressure-assisted welding has higherefficiency. This is because it promotes better surface contact as wellas it involves less complicated physical procedure; but in theEG-assisted welding methods, the welding depends on not only the BERefficiency, but also the diffusion and evaporation of EG during welding.However, it is seen that EG-assisted welding can achieve the samefracture energy level after about 2 hours. Even tests of less than 2hours, the EG-epoxy glue method demonstrates higher fracture energy thanthat in the pressure-assisted welding condition with 2 kPa applied. Infuture research, it will be interesting to further explore thedependency of solvent-driven welding performance on multiple influencingparameters in addition to the welding time and catalyst concentration.For example, for the pre-treatment method, increasing the pre-soakingtime increases the thickness of the interfacial bonding layer, whichconsequently improves the ultimate fracture energy; but the tradeoff isthat this requires more welding time to evaporate the EG solvent on theinterface. For the EG-epoxy glue method, the residual EG solvent in theepoxy glue should also be studied, as it can penetrate into the freshepoxy to be welded and promote the welding. The accomplishment of thesestudies could help to gain a more comprehensive understanding of thephysics of EG-assisted transesterification, as well as to assist in theselection of optimized conditions to improve the ultimate fractureenergy and welding speed.

Inspired by the aforementioned EG-epoxy glue method, we conductedexperiments on surface damage repair (FIGS. 11A-11B). The surface of aCAN epoxy (30 mm×12 mm×5 mm) with 5 mol % catalyst was scratched byusing a needle (FIG. 11A). Next, the damaged surface was covered by athin layer (˜0.5 mm) of dissolved epoxy solution with the same catalystcontent, following by heating at 180° C. for 3 h. FIG. 11B shows anoptical microscope image of the repaired surface, which is seen to besmooth and damage free. This ability to repair micro-cracks on a CANepoxy surface is of great interest because it could prevent the cracksfrom developing to macro-scale failure, and would broaden the servicelifetime and guarantee the safe performance of materials.

Powder-Based Reprocessing

The above-mentioned pre-treatment welding is shown to be effective toweld two bulk epoxy thermosets without the use of welding pressure. Themethod can be utilized to weld objects with more complex interfaces.Here, we demonstrate powder-based reprocessing using EG solvent byfollowing the procedure illustrated in FIG. 4. With the evaporation ofEG solvent, the partially dissolved epoxy particles gradually merge intoa solid film. The capillary force and relatively low viscosityintroduced by the EG evaporation help to maintain a glossy film surface.FIG. 12 shows images of the sample at each step, whereas FIG. 12, step(d) shows the strip of the material that was cut from the whole film fora better demonstration.

To quantitatively identify the catalytic effects on the reprocessingability of CANs, the mechanical properties of assembled CAN powders overdifferent catalyst concentrations were tested. FIG. 13A shows acomparison of the tensile stress-strain behaviors of fresh samples andreprocessed samples with different catalyst concentrations. It isobvious that with the same amount of reprocessing time (180 min), boththe elastic modulus and the ultimate strength of the reprocessed samplesare increased as the catalyst concentration increases, indicating thatthe catalyst could promote reprocessing efficiency. The mechanicalbehavior of the reprocessed sample with 7 mol % catalyst almostcompletely recovered to the values of a fresh sample.

Increasing the reprocessing time also improves the final properties ofreprocessed samples with a specific catalyst concentration. FIGS.13B-13C respectively summarize the elastic modulus (within the first ˜2%stretch) and ultimate strength of the reprocessed samples, where thereprocessing time at 180° C. is increased from 100 min to 260 min. Itcan be seen that increasing the reprocessing time is more effective inrecovering the elastic modulus than in recovering the ultimate strength.With 5 mol % catalyst, the ultimate strength is only recovered by ˜65.3%(1.24 MPa compared with 1.90 MPa of fresh epoxy) after being heated for180 min, while the modulus is recovered by ˜73.4% (2.5 MPa compared with3.4 MPa of fresh epoxy). Especially for the 1% mol catalyst, therecovery of ultimate strength is obviously slower than the recovery ofmodulus. This might be attributed to the complex interfacial behaviorsin the micro-scale.

Recycling of Epoxy Thermosets by Using Mono-Alcohol 2-Ethyl-1-Hexanol(2E1H)

The recyclability of the epoxy thermosets were also demonstrated byusing a mono-alcohol 2-ethyl-1-hexanol (2E1H), which is shown in FIG.15A. The epoxy material was seen to be fully dissolved in an enclosed(sealed) environment after being heated at 180° C. for ˜20 h. Therepolymerization was performed in an open-air environment to facilitatethe evaporation of 2E1H. After being heated at 200° C. for ˜12 h (withmagnetic stirring) and then at 180° C. for ˜42 h, the dissolved polymersolution was repolymerized. Compared with the recycling with EG solvent,the recycling efficiency with mono-alcohol 2E1H is much lower. This isbecause that each 2E1H molecule only provides one hydroxyl group, whichconsequently leads to a lower BER efficiency compared with that usingdi-functional alcohol EG.

FIG. 15B compares the glass transition behavior of fresh epoxy andrecycled epoxy by using EG and 2E1H solvents, where all the three epoxysamples exhibit similar transition temperature and storage moduluswithin the tested temperature range. Specifically, the storage modulusof the recycled epoxy is slightly higher than that of fresh sample,which might be resulted from the oxidation of epoxy during therepolymerization. This observation is also consistent with previousstudies on the effect of inherent thermo-oxidation of epoxy materials.However, since the modulus change is small, we expected a minimalinfluence of oxidation on the BER kinetics and associatedthermomechanical properties of epoxy thermosets.

FTIR tests were also performed to examine the conversions of functionalgroups (esters, hydroxyls, and epoxy COC groups) during the 2E1Hassisted recycling process. Specifically, the fully recycled epoxythermoset was cut in the middle, and FTIR scan was performed on thecross-section to probe the chemical composition inside the material. Theresults are compared with those in EG assisted recycling process inFIGS. 16A-16B. As shown in the figures, both ester and hydroxyl groupsin the recycled epoxy return to the same level as that of fresh epoxy,which indicates that there is no remnant 2E1H molecules left in thewhole epoxy network, and the dissolved polymer solution is fullypolymerized.

Conclusion

A pressure-free, solvent driven surface welding technique for an epoxybased covalent adaptable network (CAN) is demonstrated. This method usesethylene glycol (EG) to participate in the bond exchange reactions(BERs) and to tune the integrity of the network. When EG is present inabundance, the EG molecules participate in BERs, and effectively breakthe long polymer chains into small chain segments, namely a dissolutionprocess; when EG is under evaporation condition, the EG molecules areregenerated and escape, resulting in repolymerization of the CAN epoxy.When such dissolution and repolymerization processes occur on a materialinterface, they offer a way to realize pressure free surface welding andpowder-based reprocessing. We first tested dissolution rate as afunction of time, temperature, catalyst concentration and amount of EG.We found that the catalyst concentration defines the initiating point ofdegradation, while its rate depends on the temperature as well as thediffusion rate of the dissolved polymer chains. Thermomechanicalcharacterization was performed to verify that the properties ofrepolymerized epoxy are very close to those of fresh one. We alsodeveloped two solvent driven surface welding techniques: thepre-treatment method and the EG-epoxy glue method. It was shown that theinterfacial fracture energy can eventually reach the same level as thatobtained in the pressure-assisted surface welding method. In addition,the EG-epoxy welding mechanism was extended to repair the epoxythermosets with surface damage. Finally, the EG-assistedtransesterification method was utilized to reprocess CAN epoxy frompowders. We found that the elastic modulus of the reprocessed sample canbe fully recovered after it has been heated for a sufficiently longtime. These exciting explorations open new possibilities in applicationof thermosetting polymers, such as convenient healing orreprocessability in a more available and practical way. In welding andpowder-based reprocessing, increasing the concentration of catalyst canincrease the efficiency of welding and reprocessing.

Example 2: Carbon Fiber Reinforced Thermoset Composite with Near 100%Recyclability

An environmentally and economically favorable recycling method isdemonstrated for CFRP composite, which takes advantage of the dynamicnature of recently emerged CANs. Briefly, thermosetting polymers capableof transesterification type bond exchange reactions (BERs) are seen tobe fully dissolved in alcohol solvents at relatively low temperature(160° C. 180° C.). Further heating the dissolved polymer solution leadsto the evaporation of residual alcohol solvents, and repolymerization ofthe thermoset with the near-identical thermomechanical properties asfresh polymers. Based on this, when the CANs are used as compositebinders, we are able to reclaim both thermoset matrix and carbon fiberwith their original properties undiminished, to repair the polymermatrix in a CFRP composite, as well as to fully recycle the CFRP byusing the reclaimed fiber and dissolved polymer solution. Such arecycling paradigm is advantageous by virtue of its low cost, easyimplementation, and almost 100% recyclability.

Experimental Section

Materials

The epoxy based CAN is cross-linked by fatty acid to enable thetransesterification type BERs. It was synthesized by using diglycidylether of bisphenol A (DGEBA, Sigma Aldrich, St. Louis, Mo., USA), fattyacids (Pripol 1040, Uniqema Inc., Paterson, N.J., USA), and metalcatalyst (Zn(Ac)₂, Sigma Aldrich, St. Louis, Mo., USA). The carbon fiberused is a plain weave fabric (Fibre Glast Developments Corp.,Brookville, Ohio), with tensile strength from 4.2 GPa to 4.4 GPa, andtensile modulus from 227.5 GPa to 240.6 GPa. For the dissolution ofthermosetting polymers, anhydrous ethylene glycol (EG, Sigma Aldrich,St. Louis, Mo., USA) was used in this study, with a purity of 99.8% anda boiling point at 197.3° C.

Synthesis of the Epoxy Thermoset and its Composite

In the previous work of Leibler and coworkers, the synthesis method ofepoxy thermosets was demonstrated (Montarnal, D., et al., Science, 2011.334(6058): p. 965-968.). In this example, we adopt the same method tofabricate their composite material: In the first step, Zn(Ac)₂ catalyst(263.61 g/mol) was mixed with fatty acids (296 g/mol) in a beaker. Themole ratio between catalyst and COOH groups is 0.05:1. The mixture wasthen transferred into an oven at 180° C. and heated under vacuum.Catalyst particles were fully solubilized in fatty acids when no gasevolution was observed. This typically takes 2-3 h. Subsequently, meltedDGEBA was added to the mixture and manually stirred until the mixturebecame homogeneous and translucent. The mole ratio between epoxy groupsin DGEBA and COOH groups is 1:1. After this, the mixture was placed invacuum to remove the bubbles. Finally, the mixture was poured into amold, with carbon fiber fabric laid in the middle (the fiber will beremoved when the pure epoxy is prepared). After being heated at 130° C.for 6 h, the epoxy matrix can be fully cured. Throughout this study, theweight fraction of carbon fiber in the composite is always 13.7 wt %

Uniaxial Tension Tests

A uniaxial tension test was adopted to evaluate the mechanicalproperties of the fresh, repaired and recycled CFRP composites. All thesamples were cut to the same dimension (87.0 mm×10.8 mm×1.88 mm). Thetests were performed on an MTS Universal Materials Testing Machine witha load capacity of 10 kN (Model Insight 10). During the room-temperatureuniaxial tension test, the separation rate of the clamping heads is 5mm/min for all cases.

Stress Relaxation Tests

The time and temperature dependent stress relaxation behavior of freshand repolymerized epoxy thermosets was tested by a DMA tester (ModelQ800, TA Instruments, New Castle, Del., USA). Samples with the samedimensions (15.0 mm×4.0 mm×1.0 mm) were first preloaded by a 1×10⁻³Nforce to ensure straightness. After reaching the testing temperature,they were allowed 30 min for thermal equilibrium. The specimens werethen stretched by 1% on the DMA machine and the deformation wasmaintained during the test. The decrease of stress was recorded and thestress relaxation modulus was calculated.

Glass Transition and Stress-Strain Behavior of Repolymerized Epoxy

The glass transition of the epoxy thermoset was tested by using adynamic mechanical analysis (DMA) tester (Model Q800, TA Instruments,New Castle, Del., USA). During the DMA test, polymer sheet was firstheated and then held at 100° C. for 20 min to reach thermal equilibrium,followed by applying a preload of 1 kPa to maintain the sample stayingstraight. During the experiment, the strain was oscillated at afrequency of 1 Hz with a peak-to-peak amplitude of 0.1% while thetemperature was decreased from 100° C. to −50° C. at a rate of 1° C.min⁻¹. Once the temperature reached −50° C., it was maintained for 30min and then increased to 100° C. at the same rate. This procedure wasrepeated for multiple times and the data from the last cooling step isreported. The DMA tester was used to carry out room-temperature uniaxialtension tests for the repolymerized epoxy. The loading rate was chosento be a small value (5%/min for all tests) to minimize viscoelasticeffects. FIG. 21A shows the glass transition behavior of both fresh andrepolymerized epoxy, where the normalized tan δ and storage modulus areplotted as a function of temperature. The temperature corresponding tothe peak of tan δ is referred as the glass transition temperature T ofthe material. The T of the repolymerized epoxy is 33.2° C., which isalmost same with that fresh CAN epoxy. FIG. 21B shows the stress-strainbehavior. The results also show that the repolymerized epoxy has nearlythe same stress-strain behavior as that of the fresh epoxy.

Results and Discussions

The recycling method for CFRPs was demonstrated using an epoxy basedthermosetting polymer. FIG. 17A depicts the formation of fatty acidlinkers on the backbone of the polymer chains during polymerization,where the fatty acid linker is the reaction derivative from theepoxy/fatty acid reaction. In the previous example, the material'sthermomechanical properties and malleability were tested. It was shownthat at 180° C., which is ˜150° C. above the glass transitiontemperature T_(g) (˜30° C.), the epoxy can relax the internal stress by80% within 30 min due to the transesterification BERs. Yet at lowtemperature, the epoxy behaves like traditional thermosetting polymers.Since the alcohol solvent is expected to participate in the BERs at hightemperature, we will use ethylene glycol (EG) due to its relatively highboiling point (197.3° C.)

Dissolution and Repolymerization of Epoxy thermosets

The developed method for recycling CFRPs relies on the dissolution andrepolymerization of epoxy thermoset in the EG solvent viatransesterification at high temperature; it is illustrated in FIG. 17B.Transesterification includes the process of exchanging the organic groupof an ester with the organic group of an alcohol. When a piece of CANepoxy is immersed in the solvents, the hydroxyl group in EG participatesin transesterification reactions; since EG molecules are small and arenot linked to long chain polymers, they effectively break the longpolymer chains into small sections. It should be noted that the reversereaction, where an EG molecule is regenerated (i.e. the repolymerizationreaction in FIG. 17B), can also occur; but when EG is in excess amount,the dissolution reaction dominates. Therefore, when sufficient EGsolvent is provided, the dissolution event starts at the polymer surfaceand proceeds as the broken chain segments diffuse away from thepolymer-solvent interface. Eventually, the epoxy network can be fullydissolved. However, this will change if the reactions are conducted inan environment where EG solvent tends to evaporate. There, the EGsolvent will leave the solution, leaving the repolymerization reactionshown in FIG. 17B to dominate. This means that the dissolved thermosetnetwork will be polymerized again at a higher temperature when the EGmolecules are gradually evaporated out.

While it is straightforward to conjecture that a full dissolution can beeventually realized with sufficiently long period of soaking in asufficient amount of alcohol solvent, we are interested in examining howfast a given piece of CAN could be dissolved and the minimum amount ofEG solvent we should use. The dissolution rate of epoxy thermosets inthe EG solvent is determined by the following processes: the diffusionof EG molecules into the polymer network, the breaking and reforming ofpolymer chains due to the transesterification, and the diffusion ofbroken chain segments into the solvent. While the kinetics of all thesethree processes depend on the heating time and temperature, thediffusion processes also depend on the sample's dimensions. As astarting point, all the epoxy samples under test have the same dimension(11.2 mm×6 mm×3 mm). Besides, the dissolution temperature is set to be180° C. in all cases. The temperature dependent dissolution rate can beeasily extrapolated according to the kinetics of the aforementionedprocesses. For example, it is well known that the diffusivity in solidsand liquids can be well predicted by the Arrhenius and theStokes-Einstein equations respectively. The kinetics of thetransesterification BER follows an Arrhenius type time-temperaturesuperposition.

Two groups of dissolution tests were performed. In the first group ofexperiments, epoxy samples with 5 mol % catalyst (see the MaterialsSection for details) were immersed in 3 g EG solvents and their weightswere measured at different time points. To avoid the evaporation of EG,the container was sealed when it was heated at high temperature. FIG.17C plots the average weight of the epoxy samples (normalized by theinitial weight of epoxy) as a function of soaking time in EG at 180° C.It is seen that the epoxy block starts to dissolve at ˜25 min of soakingand can be fully dissolved after ˜160 min. In contrast, if there is noBER involved in the epoxy network, for example if no catalyst waspresent or if the test was conducted at low temperature (25° C.), nodissolution was observed in the samples even after being immersed in thesolvent for 4 hours. Specifically, the epoxy sample without catalyst wasseen to swell ˜4 wt % of EG after being heated at 180° C. for 4 hours.

In the second group of experiments, epoxy samples were immersed indifferent amounts of EG solvent, while the soaking time was set to be 4h in each case. FIG. 17D shows the normalized weight of epoxy as afunction of EG weight (normalized by the initial weight of epoxy). It isseen that the minimum amount of EG for a full dissolution of 1 g epoxyis ˜0.4 g. This is consistent with the following stoichiometryconsideration: For 1 g of epoxy with 5.59×10⁻³ mol ester group, thereshould be at least 5.59×10⁻³ mol EG molecules to break all the polymerchains for full dissolution, which equals to 0.342 g.

As illustrated in FIG. 17B, the depolymerized thermoset network can bepolymerized again when the EG molecules are allowed to evaporate. Inthis study, we focus on the thermomechanical properties of repolymerizedepoxy, and compare them with those of fresh material. During theexperiments, the dissolved polymer solution was heated at 180° C. foranother 10 h, where the container was unsealed to allow the evaporationof EG. FIGS. 21A-21B show the comparison of glass transition andstress-strain behavior between the fresh and repolymerized epoxy. FIG.17E shows the BER-induced stress relaxation behaviors at differenttemperatures (120° C., 140° C., 160° C., 180° C. and 200° C.respectively). The results show that the repolymerized epoxy has nearlythe same thermomechanical properties and malleability as those of thefresh epoxy. The epoxy can be fully recycled by using the EG solvent.

Repairing of Composite

In the CFRPs, polymer matrix is mainly used to maintain the shape ofcomposites, transfer load to the embedded carbon fiber, and protect thefiber from the environment. Although fiber damage is possible (such asat the site of impact), matrix damage in the form of delaminations,cracks, surface abrasion and corrosion is more common in engineeringapplications. Traditional methods to repair the polymer matrix of CFRPs,such as the patch repair, scarf repair and resin injection, usuallyrequire high skill and might change the weight and geometry of thecomposite. Besides, they are not efficient for repairing damage in theform of cracks that form deep within the structure where detection isdifficult and repair is almost impossible. Recent advances in CANs openan avenue to develop self-healing thermosetting polymers, where polymerchains can be re-connected on the interface for welding. This furtherenables the full repair of thermosetting polymers. For example in ourprevious work, polymer powder was applied to repair broken thermosettingpolymers (Yu, K., et al., RSC Advances, 2014. 4(20): p. 10108-10117).However, for these self-healing materials, pressure is necessary toguarantee a good contact of interfaces during the repair, which would bechallenging in most engineering applications, especially when the CFRPsare in service.

Here, we demonstrate the repairability of the CFRP composite based onthe mechanism of polymer dissolution and repolymerization, where onlyheating is required to fully repair the composite while obtaining thesame dimensions and mechanical properties. The repairing procedure isillustrated in FIG. 18A. Initially, a fresh composite was manuallyscratched on the polymer surface to render a damage area. The weightloss of the CFRP was recorded. Then the sample was embedded into a moldthat fit the composite's dimensions. The damaged surface was covered byCAN powder whose weight equaled the weight lost during the scratching.Sufficient EG solvent was instilled into the mold, and the temperaturewas ramped to induce transesterifications. During heating, the polymerpowder was be dissolved, diffused together and repolymerized with thepolymer matrix. Eventually, the damaged area on the composite surfacewas repaired. It should be noted that in this study, the compositerepair only involves the repair of the polymer matrix, while theembedded carbon fiber is intact. We start with the CFRP composite withsurface damages. The repair of damages in the form of delaminations andcracks can be readily implemented.

FIG. 18B shows experimental pictures obtained during the compositerepair. The initial fresh composite sample (25.4 mm×10.8 mm×1.88 mm)consists of ˜0.66 g epoxy matrix and ˜0.11 g carbon fiber. When manuallyscratching the composite surface on sandpaper, caution was taken toavoid any damages to the carbon fiber fabric. After scratching, theweight loss of the composite was determined to be ˜0.16 g. The compositewas then transferred to a mold, and 0.16 g (same amount as the weightloss) of epoxy powder, with an average size of ˜20 um to ˜120 um, wasrefilled to cover the damaged surface. According to the analysis inSection 2.1, at least ˜0.055 g EG solvent (34.2 wt % of the epoxy) wasneeded to dissolve the refilled epoxy powder, and ˜0.23 g was needed todissolve all the epoxy material in the mold. Indeed, for good repairperformance, in addition to fully dissolving the powder, excessive EGsolvent is necessary to depolymerize the surface layer of the damagedCFRP, so the repolymerized epoxy can bond well with the remnant bulkepoxy. However, it is not necessary to dissolve all the epoxy materialin the mold for the purpose of repair. In our experiments, 0.1 g, 0.15g, and 0.2 g EG solvent was respectively instilled into the mold, andFIG. 18B shows the case of 0.15 g EG being applied. Subsequently, themold was covered by a glass slide to prevent evaporation of the EGsolvent. After being heated at 180° C. for 4 hours, which is asufficiently long time to dissolve the epoxy powder (according to FIG.17C), the glass slide was removed to facilitate evaporation andrepolymerization. Finally, the CFRP composite with surface damage wasfully repaired after being heated for another 10 hours,

In our demonstrated repair routine, the application of EG avoids the useof pressure when repairing the CFRP composites, which is easier toimplement in real engineering applications. The mechanical properties ofthe repaired composite were examined by using uniaxial tension tests onthe MTS machine. We observed that the three repaired composites withdifferent amounts of EG being applied exhibit almost identicalmechanical properties. The stress-strain curves of the repaired CFRPswill be compared with those of fresh composites and recycled compositesas shown in the next section.

Recycling the CFRP Composite

The closed-loop recycling paradigm of the CFRP composite is illustratedin FIG. 19A. A fresh composite is first bathed in EG solvent at hightemperature. The matrix epoxy is gradually dissolved, and the cleanfiber fabric can be separated. Subsequently, the polymer solution isfurther heated to evaporate the EG solvent. Since the alcohol solventand dissolved polymers are not compatible, it is easy to inspect theamount of excessive EG. Finally, both the reclaimed fiber fabric and thepolymer solution are put into a mold that fits with the fiber fabricdimension. A new composite is remanufactured after repolymerizing thepolymer solution.

FIGS. 19B-19C show pictures obtained during an experiment that followsthe procedure illustrated in FIG. 19A. The initial fresh composite has adimension of 27.1 mm×10.8 mm×1.88 mm, and consists of 0.7 g epoxy matrixand 0.118 g carbon fiber. After immersing the composite in 20.9 g EGsolvent at 180° C. for 4 h, the matrix was fully dissolved, and theclean carbon fiber was reclaimed. Further heating the dissolved polymersolution for another 3 hours evaporated the excessive EG solvent (seeFIG. 19B). Subsequently, this polymer solution was transferred into amold with the reclaimed fiber laid in the middle, as shown in FIG. 19C.After being heated at 180° C. for ˜10 hours, the epoxy wasrepolymerized, and the first generation recycled CFRP composite wasfabricated. The weight of the recycled composite is 0.805 g, which isslightly smaller than that of the fresh one (0.818 g) because of loss ofepoxy resin during the transfer. This recycling procedure was repeatedfor multiple times and the stress-strain behavior of each generation ofrecycled CFRP composite was measured on the MTS machine at roomtemperature.

The microscale morphology of both fresh and reclaimed fiber was observedby using a Scanning Electron Microscope (SEM, Model Phenom Pro,PhenomWorld, Netherlands), and the imagines are shown in FIGS. 20A-20B.Before the SEM, the fiber fabric was coated with a thin gold film on aSputter Coater (Mode 108 Auto, Cressington Scientific Instruments Ltd.,Watford, UK). The thickness of the gold film was around 100 nm. As shownin the figure, the reclaimed carbon fiber retains the same fabricpattern as the original one. Besides, there is no visible damage oralternation in fiber dimension. It is also seen that some residualpolymer solution is attached on the fabric surface, so the weight of thereclaimed fiber (0.121 g) is slightly higher than that embedded in thefresh composite (0.118 g).

Uniaxial tension tests were applied to examine the mechanicalproperties, such as modulus and strength, of both fresh and reclaimedcarbon fiber. The experimental results are shown in FIG. 20C. Here, allthe fiber fabrics were cut into the same dimension (115.6 mm in lengthand 22.3 mm in width), with 11 bundles along the stretching direction.Since the fiber fabrics are plain woven ones, we expect minimuminfluence on the uniaxial tension behavior from the fiber bundlesvertical to the stretching direction.

The tests were performed on the MTS Machine with a load capacity of 10kN, where the fiber fabric was stretched at a constant displacement rateuntil it fully broke. The separation rate of the clamps was set to be2.3 mm/s in each case, and the corresponding strain rate was 2.4%/s. Theinset images of FIG. 20C show the appearance of the fiber fabric beforeand after tests. Specifically, the two ends of the fiber fabrics wereembedded in epoxy matrix (˜3 mm in thickness), which facilitates easyclamping on the MTS machine. The average stress in each carbon fiber canbe estimated according to the fabric architecture: there are 1000 T300carbon fibers in each fiber bundle, and the diameter of each fiber is 7um.

The typical stress-strain curves of both fresh and reclaimed fiber areshown in FIG. 20C. Two samples from fresh fibers and from reclaimedfibers are shown in FIG. 20C, respectively. As the fiber fabricstretches, the stress is seen to ramp linearly with strain until acritical value is reached. Then the stress drops dramatically, whichindicates the breaking of a majority of carbon fibers. It is shown thatthe stress-strain curves of both fresh and reclaimed fiber fabrics agreewell. For the reclaimed carbon fiber, the measured modulus (223 GPa±16.3GPa) and strength (4.4 GPa±0.53 GPa) is very close to those of freshones (239 GPa±11.7 Gpa, 4.2 GPa±0.19 Gpa respectively), i.e. thereclaimed fibers retained 97% of the modulus and 95% of the tensilestrength of the original fibers. The results in FIGS. 20A-20Cdemonstrate that the reclaimed carbon fiber retains the same dimensionand mechanical properties, which results from the friendly operationalconditions being applied during recycling: the applied EG solvent isnon-corrosive to the carbon fiber even at high temperatures. Inaddition, the recycling temperature (180° C.) is far below the thermaldecomposition temperature of T300 carbon fiber that was reported before(up to 500° C.), below which the fiber still retains excellentmechanical and dimensional stability.

FIG. 20D plots the room-temperature stress-strain curves of freshcomposite, first generation recycled composite, as well as the repairedcomposite discussed in Section 2.2. The good consistency shown in thefigure indicates that the mechanical properties of the repaired andrecycled CFRP can be fully recovered. Specifically, since the ultimatestrength of CFRP composite is mainly determined by the carbon fiber, theagreement in the material strength further proves that the carbon fiberfabric remains intact during the recycling. Besides, a single piece ofCFRP composite can be recycled for multiple times. As shown in FIG. 20E,each generation of recycled composite maintains the same level of theelastic modulus (within the first 2% stretch) and ultimate strength,which indicates good repeatability of the developed recycling method.

DISCUSSION

An environmentally and economically favorable recycling method for theCFRP composites is presented. This approach has the advantage of lowcost, easy implementation, zero pollution and full recyclability;therefore, it will greatly facilitate waste management and environmentalprotection.

The CFRP composite studied in this example is flexible due to the lowtransition temperature of the epoxy matrix (T_(g)=30° C.). However, inmany engineering applications, stiffer composites are preferred asstructural materials. The method can be applied to recycle other typesof epoxy composites with different thermomechanical properties. Indeed,the possibility can be easily demonstrated after considering previousworks on the CAN epoxy, where fatty acid is replaced by the glutaricanhydride to crosslink the network. Since the anhydride hard linker hasshorter backbone chains of carbon atoms than the fatty acid linker shownin FIGS. 17A-17B, the resulting network is stiffer with higher modulus,and the T_(g) is elevated to ˜63° C. Even though the thermomechanicalproperties are tuned, the underlying dynamic chemistry is unchanged, andthe network is still capable of transesterification type BERs. In termsof this, when using such stiffer epoxy as binder, the CFRP composite canstill be fully recycled and repaired by using the developed method.

In addition to the ester-containing epoxy, the recycling paradigm can bereadily extended to other types of thermoset, as long as a proper smallmolecule solvent is chosen to break the polymer chains from backbone,and to subsequently evaporate away during the repolymerization step. Forexample, epoxy thermosets with dynamic C—S bonds can be dissolved byusing a thiol-containing solvent (2-mercaptoethanol solvent). Polyimidethermosets with dynamic C—N bonds can be fully dissolved and recycledusing amine solvent.

CONCLUSIONS

A method to fully recycle fiber reinforced epoxy composites isdemonstrated. After immersing the composite in solvent, small moleculeswith proper functional groups will participate in BERs with the CANmatrix upon stimuli, and break the long polymer chains into smallsegments. The resulted clean fibers, with the same dimensions andmechanical properties as fresh ones, can be reclaimed after the polymermatrix is fully dissolved. The reclaimed fibers retain 97% of themodulus and 95% of the tensile strength of the original fibers. Both therecycled fiber and the dissolved polymer solution can be reused tofabricate a new generation of composite, which enables a closed-loopnear 100% recycling paradigm. We demonstrated the effectiveness of therecycling method by using an epoxy based CFRP. The dissolution rate ofthis epoxy in EG solvent was first examined, and the minimum amount ofEG to fully dissolve the epoxy was revealed. By using the EG solvent,the epoxy based CFRP was shown to be fully repaired and recycled.Compared with traditional recycling methods, this new method mainlyinvolves simple heating and usage of a proper solvent that isenvironmentally friendly and easy to manage. In addition, there is nodamage in either the epoxy or the fiber during the recycling. Therecycled and repaired composite can exhibit the same or nearly the samemechanical properties as fresh ones.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations, andare set forth only for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiments of the disclosure without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure.

1-22. (canceled)
 23. A method of recycling a thermoset polymer or acomposite thereof, wherein the thermoset polymer or composite thereofcomprises thermoset polymer matrix having a plurality of ester bonds,the method comprising: (A) washing the thermoset polymer or a compositethereof in a small molecule alcohol in the presence of a catalyst at afirst elevated temperature for a first period of time sufficient todissolve the thermoset polymer matrix; and ((B) evaporating the smallmolecule alcohol at a second elevated temperature for a second period oftime to re-polymerize the thermoset polymer matrix. wherein the smallmolecule alcohol has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of ester bonds.
 24. Themethod according to claim 23, comprising recycling of a thermosetpolymer composite, wherein the thermoset polymer composite comprises areinforcing material dispersed within the thermoset polymer matrix. 25.The method according to claim 24, wherein the reinforcing materialcomprises a material selected from the group consisting of a glassfiber, a carbon fiber, an aramid fiber, a boron fiber, a graphite, and acombination thereof.
 26. The method according to claim 24, wherein thereinforcing material has a thermal decomposition temperature of about500° C. or less.
 27. The method according to claim 24, wherein thereinforcing material has a structure selected from the group consistingof a continuous fiber, a cloth, a fabric, a yarn, and a tape.
 28. Themethod according to claim 24, wherein the small molecule alcohol has aboiling point below a thermal decomposition temperature of thereinforcing material.
 29. The method according to claim 24, wherein themethod comprises reclaiming the reinforcing material from the thermosetpolymer matrix prior to re-polymerizing the thermoset polymer matrix.30. The method according to claim 29, wherein the reclaimed reinforcingmaterial has an elastic modulus that is within 97% of an elastic modulusof the reinforcing material prior to the recycling and when measuredunder the same conditions.
 31. The method according to claim 29, whereinthe reclaimed reinforcing material has a tensile strength that is within95% of a tensile strength of the reinforcing material prior to therecycling and when measured under the same conditions.
 32. The methodaccording to claim 24, wherein the small molecule alcohol has is apolyol having a boiling point of about 160° C. to 180° C.; wherein thebond exchange reaction (BER) activation temperature is about 120° C. orhigher; wherein the thermoset polymer matrix comprises an anhydridecured epoxy, an unsaturated polyester, or a combination thereof; whereinone or both of the first elevated temperature and the second elevatedtemperature are independently about 160° C. to 200° C.; wherein thecatalyst is a transesterification catalyst selected from the groupconsisting of a lead oxide, a lead sulfide, a lead hydroxide, aplumbite, a plumbate, a lead carbonate, a copper compound, a silvercompound, a gold compound, a zinc compound, a cadmium compound, an ironcompound, a cobalt compound, a salt thereof, and a combination thereof;wherein the catalyst is present in an amount from about 2 mol-% to about10 mol-%. 33-44. (canceled)
 45. A method of repairing a surface of athermoset polymer or a composite thereof, wherein the thermoset polymeror composite thereof comprises thermoset polymer matrix having aplurality of ester bonds, and wherein the surface comprises animperfection; the method comprising: (A) applying a powder of thethermoset polymer to the surface of the thermoset polymer or compositethereof, (B) contacting the surface of the thermoset polymer orcomposite thereof and the powder with a small molecule alcohol in thepresence of a catalyst at a first elevated temperature for a firstperiod of time sufficient to dissolve the powder and at least a portionof the thermoset polymer matrix at the surface of the thermoset polymeror composite thereof, and (C) evaporating the small molecule alcohol ata second elevated temperature for a second period of time sufficient tore-polymerize the thermoset polymer matrix incorporating the thermosetpolymer from the powder to repair the imperfection; wherein the smallmolecule alcohol has a boiling point above a bond exchange reaction(BER) activation temperature for the plurality of ester bonds.
 46. Themethod according to claim 45, comprising repairing a surface of athermoset polymer composite, wherein the thermoset polymer compositecomprises a reinforcing material dispersed within the thermoset polymermatrix.
 47. The method according to claim 46, wherein the reinforcingmaterial comprises a material selected from the group consisting of aglass fiber, a carbon fiber, an aramid fiber, a boron fiber, a graphite,and a combination thereof.
 48. The method according to claim 46, whereinthe reinforcing material has a thermal decomposition temperature ofabout 500° C. or less.
 49. The method according to claim 46, wherein thereinforcing material has a structure selected from the group consistingof a continuous fiber, a cloth, a fabric, a yarn, and a tape. 50-63.(canceled)
 64. A method of chemically welding a first surface to asecond surface, wherein both the first surface and the second surfacecomprise a thermoset polymer matrix having a plurality of ester bonds,the method comprising: (A) contacting the first surface and the secondsurface with a small molecule alcohol in the presence of a catalyst at afirst elevated temperature for a first period of time; (B) contactingthe first surface and the second surface to form an interface; and (C)evaporating the small molecule alcohol to polymerize the thermosetpolymer matrix at the interface; wherein the small molecule alcohol hasa boiling point above a bond exchange reaction (BER) activationtemperature for the plurality of ester bonds.
 65. The method accordingto claim 64, wherein the interface has an interfacial fracture energy ofabout 1200 J/m² to 5000 J/m².
 66. The method according to claim 64,wherein the small molecule alcohol has a boiling point of about 160° C.to 180° C.
 67. The method according to claim 64, wherein the smallmolecule alcohol is a polyol.
 68. The method according to claim 64,wherein the small molecule alcohol is selected from the group consistingof a diol, a triol, and a polyol. 69-200. (canceled)